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/service.rs

@@ -7,7 +7,7 @@ use crate::{
     subnet_service::{AttestationService, SubnetServiceMessage},
     NetworkConfig,
 };
-use beacon_chain::{BeaconChain, BeaconChainError, BeaconChainTypes};
+use beacon_chain::{BeaconChain, BeaconChainTypes};
 use futures::channel::mpsc::Sender;
 use futures::future::OptionFuture;
 use futures::prelude::*;
@@ -30,8 +30,8 @@ use task_executor::ShutdownReason;
 use tokio::sync::mpsc;
 use tokio::time::Sleep;
 use types::{
-    ChainSpec, EthSpec, ForkContext, RelativeEpoch, Slot, SubnetId, SyncCommitteeSubscription,
-    SyncSubnetId, Unsigned, ValidatorSubscription,
+    ChainSpec, EthSpec, ForkContext, Slot, SubnetId, SyncCommitteeSubscription, SyncSubnetId,
+    Unsigned, ValidatorSubscription,
 };
 
 mod tests;
@@ -706,29 +706,12 @@ impl<T: BeaconChainTypes> NetworkService<T> {
 
     fn update_gossipsub_parameters(&mut self) {
         if let Ok(slot) = self.beacon_chain.slot() {
-            if let Some(active_validators) = self
+            let active_validators_opt = self
                 .beacon_chain
-                .with_head(|head| {
-                    Ok::<_, BeaconChainError>(
-                        head.beacon_state
-                            .get_cached_active_validator_indices(RelativeEpoch::Current)
-                            .map(|indices| indices.len())
-                            .ok()
-                            .or_else(|| {
-                                // if active validator cached was not build we count the
-                                // active validators
-                                self.beacon_chain.epoch().ok().map(|current_epoch| {
-                                    head.beacon_state
-                                        .validators()
-                                        .iter()
-                                        .filter(|validator| validator.is_active_at(current_epoch))
-                                        .count()
-                                })
-                            }),
-                    )
-                })
-                .unwrap_or(None)
-            {
+                .canonical_head
+                .cached_head()
+                .active_validator_count();
+            if let Some(active_validators) = active_validators_opt {
                 if self
                     .libp2p
                     .swarm
@@ -742,6 +725,14 @@ impl<T: BeaconChainTypes> NetworkService<T> {
                         "active_validators" => active_validators
                     );
                 }
+            } else {
+                // This scenario will only happen if the caches on the cached canonical head aren't
+                // built. That should never be the case.
+                error!(
+                    self.log,
+                    "Active validator count unavailable";
+                    "info" => "please report this bug"
+                );
             }
         }
     }