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>

common/task_executor/Cargo.toml

@@ -5,7 +5,7 @@ authors = ["Sigma Prime <contact@sigmaprime.io>"]
 edition = "2021"
 
 [dependencies]
-tokio = { version = "1.14.0", features = ["rt-multi-thread"] }
+tokio = { version = "1.14.0", features = ["rt-multi-thread", "macros"] }
 slog = "2.5.2"
 futures = "0.3.7"
 exit-future = "0.2.0"

common/task_executor/src/lib.rs

@@ -7,6 +7,8 @@ use slog::{crit, debug, o, trace};
 use std::sync::Weak;
 use tokio::runtime::{Handle, Runtime};
 
+pub use tokio::task::JoinHandle;
+
 /// Provides a reason when Lighthouse is shut down.
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub enum ShutdownReason {
@@ -312,6 +314,61 @@ impl TaskExecutor {
         Some(future)
     }
 
+    /// Block the current (non-async) thread on the completion of some future.
+    ///
+    /// ## Warning
+    ///
+    /// This method is "dangerous" since calling it from an async thread will result in a panic! Any
+    /// use of this outside of testing should be very deeply considered as Lighthouse has been
+    /// burned by this function in the past.
+    ///
+    /// Determining what is an "async thread" is rather challenging; just because a function isn't
+    /// marked as `async` doesn't mean it's not being called from an `async` function or there isn't
+    /// a `tokio` context present in the thread-local storage due to some `rayon` funkiness. Talk to
+    /// @paulhauner if you plan to use this function in production. He has put metrics in here to
+    /// track any use of it, so don't think you can pull a sneaky one on him.
+    pub fn block_on_dangerous<F: Future>(
+        &self,
+        future: F,
+        name: &'static str,
+    ) -> Option<F::Output> {
+        let timer = metrics::start_timer_vec(&metrics::BLOCK_ON_TASKS_HISTOGRAM, &[name]);
+        metrics::inc_gauge_vec(&metrics::BLOCK_ON_TASKS_COUNT, &[name]);
+        let log = self.log.clone();
+        let handle = self.handle()?;
+        let exit = self.exit.clone();
+
+        debug!(
+            log,
+            "Starting block_on task";
+            "name" => name
+        );
+
+        handle.block_on(async {
+            let output = tokio::select! {
+                output = future => {
+                    debug!(
+                        log,
+                        "Completed block_on task";
+                        "name" => name
+                    );
+                    Some(output)
+                },
+                _ = exit => {
+                    debug!(
+                        log,
+                        "Cancelled block_on task";
+                        "name" => name,
+                    );
+                    None
+                }
+            };
+            metrics::dec_gauge_vec(&metrics::BLOCK_ON_TASKS_COUNT, &[name]);
+            drop(timer);
+            output
+        })
+    }
+
     /// Returns a `Handle` to the current runtime.
     pub fn handle(&self) -> Option<Handle> {
         self.handle_provider.handle()

common/task_executor/src/metrics.rs

@@ -18,6 +18,16 @@ lazy_static! {
         "Time taken by blocking tasks",
         &["blocking_task_hist"]
     );
+    pub static ref BLOCK_ON_TASKS_COUNT: Result<IntGaugeVec> = try_create_int_gauge_vec(
+        "block_on_tasks_count",
+        "Total number of block_on_dangerous tasks spawned",
+        &["name"]
+    );
+    pub static ref BLOCK_ON_TASKS_HISTOGRAM: Result<HistogramVec> = try_create_histogram_vec(
+        "block_on_tasks_histogram",
+        "Time taken by block_on_dangerous tasks",
+        &["name"]
+    );
     pub static ref TASKS_HISTOGRAM: Result<HistogramVec> = try_create_histogram_vec(
         "async_tasks_time_histogram",
         "Time taken by async tasks",