Use async code when interacting with EL (#3244)

## Overview

This rather extensive PR achieves two primary goals:

1. Uses the finalized/justified checkpoints of fork choice (FC), rather than that of the head state.
2. Refactors fork choice, block production and block processing to `async` functions.

Additionally, it achieves:

- Concurrent forkchoice updates to the EL and cache pruning after a new head is selected.
- Concurrent "block packing" (attestations, etc) and execution payload retrieval during block production.
- Concurrent per-block-processing and execution payload verification during block processing.
- The `Arc`-ification of `SignedBeaconBlock` during block processing (it's never mutated, so why not?):
    - I had to do this to deal with sending blocks into spawned tasks.
    - Previously we were cloning the beacon block at least 2 times during each block processing, these clones are either removed or turned into cheaper `Arc` clones.
    - We were also `Box`-ing and un-`Box`-ing beacon blocks as they moved throughout the networking crate. This is not a big deal, but it's nice to avoid shifting things between the stack and heap.
    - Avoids cloning *all the blocks* in *every chain segment* during sync.
    - It also has the potential to clean up our code where we need to pass an *owned* block around so we can send it back in the case of an error (I didn't do much of this, my PR is already big enough 😅)
- The `BeaconChain::HeadSafetyStatus` struct was removed. It was an old relic from prior merge specs.

For motivation for this change, see https://github.com/sigp/lighthouse/pull/3244#issuecomment-1160963273

## Changes to `canonical_head` and `fork_choice`

Previously, the `BeaconChain` had two separate fields:

```
canonical_head: RwLock<Snapshot>,
fork_choice: RwLock<BeaconForkChoice>
```

Now, we have grouped these values under a single struct:

```
canonical_head: CanonicalHead {
  cached_head: RwLock<Arc<Snapshot>>,
  fork_choice: RwLock<BeaconForkChoice>
} 
```

Apart from ergonomics, the only *actual* change here is wrapping the canonical head snapshot in an `Arc`. This means that we no longer need to hold the `cached_head` (`canonical_head`, in old terms) lock when we want to pull some values from it. This was done to avoid deadlock risks by preventing functions from acquiring (and holding) the `cached_head` and `fork_choice` locks simultaneously.

## Breaking Changes

### The `state` (root) field in the `finalized_checkpoint` SSE event

Consider the scenario where epoch `n` is just finalized, but `start_slot(n)` is skipped. There are two state roots we might in the `finalized_checkpoint` SSE event:

1. The state root of the finalized block, which is `get_block(finalized_checkpoint.root).state_root`.
4. The state root at slot of `start_slot(n)`, which would be the state from (1), but "skipped forward" through any skip slots.

Previously, Lighthouse would choose (2). However, we can see that when [Teku generates that event](de2b2801c8/data/beaconrestapi/src/main/java/tech/pegasys/teku/beaconrestapi/handlers/v1/events/EventSubscriptionManager.java (L171-L182)) it uses [`getStateRootFromBlockRoot`](de2b2801c8/data/provider/src/main/java/tech/pegasys/teku/api/ChainDataProvider.java (L336-L341)) which uses (1).

I have switched Lighthouse from (2) to (1). I think it's a somewhat arbitrary choice between the two, where (1) is easier to compute and is consistent with Teku.

## Notes for Reviewers

I've renamed `BeaconChain::fork_choice` to `BeaconChain::recompute_head`. Doing this helped ensure I broke all previous uses of fork choice and I also find it more descriptive. It describes an action and can't be confused with trying to get a reference to the `ForkChoice` struct.

I've changed the ordering of SSE events when a block is received. It used to be `[block, finalized, head]` and now it's `[block, head, finalized]`. It was easier this way and I don't think we were making any promises about SSE event ordering so it's not "breaking".

I've made it so fork choice will run when it's first constructed. I did this because I wanted to have a cached version of the last call to `get_head`. Ensuring `get_head` has been run *at least once* means that the cached values doesn't need to wrapped in an `Option`. This was fairly simple, it just involved passing a `slot` to the constructor so it knows *when* it's being run. When loading a fork choice from the store and a slot clock isn't handy I've just used the `slot` that was saved in the `fork_choice_store`. That seems like it would be a faithful representation of the slot when we saved it.

I added the `genesis_time: u64` to the `BeaconChain`. It's small, constant and nice to have around.

Since we're using FC for the fin/just checkpoints, we no longer get the `0x00..00` roots at genesis. You can see I had to remove a work-around in `ef-tests` here: b56be3bc2. I can't find any reason why this would be an issue, if anything I think it'll be better since the genesis-alias has caught us out a few times (0x00..00 isn't actually a real root). Edit: I did find a case where the `network` expected the 0x00..00 alias and patched it here: 3f26ac3e2.

You'll notice a lot of changes in tests. Generally, tests should be functionally equivalent. Here are the things creating the most diff-noise in tests:
- Changing tests to be `tokio::async` tests.
- Adding `.await` to fork choice, block processing and block production functions.
- Refactor of the `canonical_head` "API" provided by the `BeaconChain`. E.g., `chain.canonical_head.cached_head()` instead of `chain.canonical_head.read()`.
- Wrapping `SignedBeaconBlock` in an `Arc`.
- In the `beacon_chain/tests/block_verification`, we can't use the `lazy_static` `CHAIN_SEGMENT` variable anymore since it's generated with an async function. We just generate it in each test, not so efficient but hopefully insignificant.

I had to disable `rayon` concurrent tests in the `fork_choice` tests. This is because the use of `rayon` and `block_on` was causing a panic.

Co-authored-by: Mac L <mjladson@pm.me>

beacon_node/beacon_chain/tests/sync_committee_verification.rs

@@ -46,15 +46,8 @@ fn get_valid_sync_committee_message(
     slot: Slot,
     relative_sync_committee: RelativeSyncCommittee,
 ) -> (SyncCommitteeMessage, usize, SecretKey, SyncSubnetId) {
-    let head_state = harness
-        .chain
-        .head_beacon_state()
-        .expect("should get head state");
-    let head_block_root = harness
-        .chain
-        .head()
-        .expect("should get head state")
-        .beacon_block_root;
+    let head_state = harness.chain.head_beacon_state_cloned();
+    let head_block_root = harness.chain.head_snapshot().beacon_block_root;
     let (signature, _) = harness
         .make_sync_committee_messages(&head_state, head_block_root, slot, relative_sync_committee)
         .get(0)
@@ -77,16 +70,9 @@ fn get_valid_sync_contribution(
     harness: &BeaconChainHarness<EphemeralHarnessType<E>>,
     relative_sync_committee: RelativeSyncCommittee,
 ) -> (SignedContributionAndProof<E>, usize, SecretKey) {
-    let head_state = harness
-        .chain
-        .head_beacon_state()
-        .expect("should get head state");
+    let head_state = harness.chain.head_beacon_state_cloned();
 
-    let head_block_root = harness
-        .chain
-        .head()
-        .expect("should get head state")
-        .beacon_block_root;
+    let head_block_root = harness.chain.head_snapshot().beacon_block_root;
     let sync_contributions = harness.make_sync_contributions(
         &head_state,
         head_block_root,
@@ -116,7 +102,7 @@ fn get_non_aggregator(
     harness: &BeaconChainHarness<EphemeralHarnessType<E>>,
     slot: Slot,
 ) -> (usize, SecretKey) {
-    let state = &harness.chain.head().expect("should get head").beacon_state;
+    let state = &harness.chain.head_snapshot().beacon_state;
     let sync_subcommittee_size = E::sync_committee_size()
         .safe_div(SYNC_COMMITTEE_SUBNET_COUNT as usize)
         .expect("should determine sync subcommittee size");
@@ -162,17 +148,19 @@ fn get_non_aggregator(
 }
 
 /// Tests verification of `SignedContributionAndProof` from the gossip network.
-#[test]
-fn aggregated_gossip_verification() {
+#[tokio::test]
+async fn aggregated_gossip_verification() {
     let harness = get_harness(VALIDATOR_COUNT);
     let state = harness.get_current_state();
 
-    harness.add_attested_blocks_at_slots(
-        state,
-        Hash256::zero(),
-        &[Slot::new(1), Slot::new(2)],
-        (0..VALIDATOR_COUNT).collect::<Vec<_>>().as_slice(),
-    );
+    harness
+        .add_attested_blocks_at_slots(
+            state,
+            Hash256::zero(),
+            &[Slot::new(1), Slot::new(2)],
+            (0..VALIDATOR_COUNT).collect::<Vec<_>>().as_slice(),
+        )
+        .await;
 
     let current_slot = harness.chain.slot().expect("should get slot");
 
@@ -406,7 +394,7 @@ fn aggregated_gossip_verification() {
                 valid_aggregate.message.contribution.clone(),
                 None,
                 &non_aggregator_sk,
-                &harness.chain.head_info().expect("should get head info").fork,
+                &harness.chain.canonical_head.cached_head().head_fork(),
                 harness.chain.genesis_validators_root,
                 &harness.chain.spec,
             )
@@ -474,6 +462,7 @@ fn aggregated_gossip_verification() {
 
     harness
         .add_attested_block_at_slot(target_slot, state, Hash256::zero(), &[])
+        .await
         .expect("should add block");
 
     // **Incorrectly** create a sync contribution using the current sync committee
@@ -488,17 +477,19 @@ fn aggregated_gossip_verification() {
 }
 
 /// Tests the verification conditions for sync committee messages on the gossip network.
-#[test]
-fn unaggregated_gossip_verification() {
+#[tokio::test]
+async fn unaggregated_gossip_verification() {
     let harness = get_harness(VALIDATOR_COUNT);
     let state = harness.get_current_state();
 
-    harness.add_attested_blocks_at_slots(
-        state,
-        Hash256::zero(),
-        &[Slot::new(1), Slot::new(2)],
-        (0..VALIDATOR_COUNT).collect::<Vec<_>>().as_slice(),
-    );
+    harness
+        .add_attested_blocks_at_slots(
+            state,
+            Hash256::zero(),
+            &[Slot::new(1), Slot::new(2)],
+            (0..VALIDATOR_COUNT).collect::<Vec<_>>().as_slice(),
+        )
+        .await;
 
     let current_slot = harness.chain.slot().expect("should get slot");
 
@@ -648,6 +639,7 @@ fn unaggregated_gossip_verification() {
 
     harness
         .add_attested_block_at_slot(target_slot, state, Hash256::zero(), &[])
+        .await
         .expect("should add block");
 
     // **Incorrectly** create a sync message using the current sync committee