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/network/src/sync/manager.rs

@@ -88,12 +88,12 @@ pub enum SyncMessage<T: EthSpec> {
     RpcBlock {
         request_id: RequestId,
         peer_id: PeerId,
-        beacon_block: Option<Box<SignedBeaconBlock<T>>>,
+        beacon_block: Option<Arc<SignedBeaconBlock<T>>>,
         seen_timestamp: Duration,
     },
 
     /// A block with an unknown parent has been received.
-    UnknownBlock(PeerId, Box<SignedBeaconBlock<T>>),
+    UnknownBlock(PeerId, Arc<SignedBeaconBlock<T>>),
 
     /// A peer has sent an object that references a block that is unknown. This triggers the
     /// manager to attempt to find the block matching the unknown hash.
@@ -229,17 +229,12 @@ impl<T: BeaconChainTypes> SyncManager<T> {
     /// ours that we consider it fully sync'd with respect to our current chain.
     fn add_peer(&mut self, peer_id: PeerId, remote: SyncInfo) {
         // ensure the beacon chain still exists
-        let local = match self.chain.status_message() {
-            Ok(status) => SyncInfo {
-                head_slot: status.head_slot,
-                head_root: status.head_root,
-                finalized_epoch: status.finalized_epoch,
-                finalized_root: status.finalized_root,
-            },
-            Err(e) => {
-                return error!(self.log, "Failed to get peer sync info";
-                    "msg" => "likely due to head lock contention", "err" => ?e)
-            }
+        let status = self.chain.status_message();
+        let local = SyncInfo {
+            head_slot: status.head_slot,
+            head_root: status.head_root,
+            finalized_epoch: status.finalized_epoch,
+            finalized_root: status.finalized_root,
         };
 
         let sync_type = remote_sync_type(&local, &remote, &self.chain);
@@ -379,7 +374,7 @@ impl<T: BeaconChainTypes> SyncManager<T> {
                     // advanced and will produce a head chain on re-status. Otherwise it will shift
                     // to being synced
                     let mut sync_state = {
-                        let head = self.chain.best_slot().unwrap_or_else(|_| Slot::new(0));
+                        let head = self.chain.best_slot();
                         let current_slot = self.chain.slot().unwrap_or_else(|_| Slot::new(0));
 
                         let peers = self.network_globals.peers.read();
@@ -482,11 +477,7 @@ impl<T: BeaconChainTypes> SyncManager<T> {
                     SyncMessage::UnknownBlock(peer_id, block) => {
                         // If we are not synced or within SLOT_IMPORT_TOLERANCE of the block, ignore
                         if !self.network_globals.sync_state.read().is_synced() {
-                            let head_slot = self
-                                .chain
-                                .head_info()
-                                .map(|info| info.slot)
-                                .unwrap_or_else(|_| Slot::from(0u64));
+                            let head_slot = self.chain.canonical_head.cached_head().head_slot();
                             let unknown_block_slot = block.slot();
 
                             // if the block is far in the future, ignore it. If its within the slot tolerance of
@@ -571,7 +562,7 @@ impl<T: BeaconChainTypes> SyncManager<T> {
         &mut self,
         request_id: RequestId,
         peer_id: PeerId,
-        beacon_block: Option<Box<SignedBeaconBlock<T::EthSpec>>>,
+        beacon_block: Option<Arc<SignedBeaconBlock<T::EthSpec>>>,
         seen_timestamp: Duration,
     ) {
         match request_id {
@@ -599,7 +590,7 @@ impl<T: BeaconChainTypes> SyncManager<T> {
                         batch_id,
                         &peer_id,
                         id,
-                        beacon_block.map(|b| *b),
+                        beacon_block,
                     ) {
                         Ok(ProcessResult::SyncCompleted) => self.update_sync_state(),
                         Ok(ProcessResult::Successful) => {}
@@ -621,7 +612,7 @@ impl<T: BeaconChainTypes> SyncManager<T> {
                         chain_id,
                         batch_id,
                         id,
-                        beacon_block.map(|b| *b),
+                        beacon_block,
                     );
                     self.update_sync_state();
                 }