Retrospective invalidation of exec. payloads for opt. sync (#2837)

## Issue Addressed

NA

## Proposed Changes

Adds the functionality to allow blocks to be validated/invalidated after their import as per the [optimistic sync spec](https://github.com/ethereum/consensus-specs/blob/dev/sync/optimistic.md#how-to-optimistically-import-blocks). This means:

- Updating `ProtoArray` to allow flipping the `execution_status` of ancestors/descendants based on payload validity updates.
- Creating separation between `execution_layer` and the `beacon_chain` by creating a `PayloadStatus` struct.
- Refactoring how the `execution_layer` selects a `PayloadStatus` from the multiple statuses returned from multiple EEs.
- Adding testing framework for optimistic imports.
- Add `ExecutionBlockHash(Hash256)` new-type struct to avoid confusion between *beacon block roots* and *execution payload hashes*.
- Add `merge` to [`FORKS`](c3a793fd73/Makefile (L17)) in the `Makefile` to ensure we test the beacon chain with merge settings.
    - Fix some tests here that were failing due to a missing execution layer.

## TODO

- [ ] Balance tests

Co-authored-by: Mark Mackey <mark@sigmaprime.io>

beacon_node/beacon_chain/src/test_utils.rs

@@ -432,7 +432,7 @@ where
             spec: chain.spec.clone(),
             chain: Arc::new(chain),
             validator_keypairs,
-            shutdown_receiver,
+            shutdown_receiver: Arc::new(Mutex::new(shutdown_receiver)),
             mock_execution_layer: self.mock_execution_layer,
             execution_layer_runtime: self.execution_layer_runtime,
             rng: make_rng(),
@@ -449,7 +449,7 @@ pub struct BeaconChainHarness<T: BeaconChainTypes> {
 
     pub chain: Arc<BeaconChain<T>>,
     pub spec: ChainSpec,
-    pub shutdown_receiver: Receiver<ShutdownReason>,
+    pub shutdown_receiver: Arc<Mutex<Receiver<ShutdownReason>>>,
 
     pub mock_execution_layer: Option<MockExecutionLayer<T::EthSpec>>,
     pub execution_layer_runtime: Option<ExecutionLayerRuntime>,
@@ -502,6 +502,17 @@ where
         epoch.start_slot(E::slots_per_epoch()).into()
     }
 
+    pub fn shutdown_reasons(&self) -> Vec<ShutdownReason> {
+        let mutex = self.shutdown_receiver.clone();
+        let mut receiver = mutex.lock();
+        std::iter::from_fn(move || match receiver.try_next() {
+            Ok(Some(s)) => Some(s),
+            Ok(None) => panic!("shutdown sender dropped"),
+            Err(_) => None,
+        })
+        .collect()
+    }
+
     pub fn get_current_state(&self) -> BeaconState<E> {
         self.chain.head().unwrap().beacon_state
     }