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/tests/merge.rs

@@ -13,7 +13,7 @@ fn verify_execution_payload_chain<T: EthSpec>(chain: &[ExecutionPayload<T>]) {
 
     for ep in chain {
         assert!(*ep != ExecutionPayload::default());
-        assert!(ep.block_hash != Hash256::zero());
+        assert!(ep.block_hash != ExecutionBlockHash::zero());
 
         // Check against previous `ExecutionPayload`.
         if let Some(prev_ep) = prev_ep {
@@ -40,7 +40,7 @@ fn merge_with_terminal_block_hash_override() {
         spec.terminal_total_difficulty,
         DEFAULT_TERMINAL_BLOCK,
         0,
-        Hash256::zero(),
+        ExecutionBlockHash::zero(),
     )
     .unwrap()
     .block_hash;