Add early attester cache (#2872)

## Issue Addressed

NA

## Proposed Changes

Introduces a cache to attestation to produce atop blocks which will become the head, but are not fully imported (e.g., not inserted into the database).

Whilst attesting to a block before it's imported is rather easy, if we're going to produce that attestation then we also need to be able to:

1. Verify that attestation.
1. Respond to RPC requests for the `beacon_block_root`.

Attestation verification (1) is *partially* covered. Since we prime the shuffling cache before we insert the block into the early attester cache, we should be fine for all typical use-cases. However, it is possible that the cache is washed out before we've managed to insert the state into the database and then attestation verification will fail with a "missing beacon state"-type error.

Providing the block via RPC (2) is also partially covered, since we'll check the database *and* the early attester cache when responding a blocks-by-root request. However, we'll still omit the block from blocks-by-range requests (until the block lands in the DB). I *think* this is fine, since there's no guarantee that we return all blocks for those responses.

Another important consideration is whether or not the *parent* of the early attester block is available in the databse. If it were not, we might fail to respond to blocks-by-root request that are iterating backwards to collect a chain of blocks. I argue that *we will always have the parent of the early attester block in the database.* This is because we are holding the fork-choice write-lock when inserting the block into the early attester cache and we do not drop that until the block is in the database.

beacon_node/beacon_chain/src/metrics.rs

@@ -240,6 +240,14 @@ lazy_static! {
     pub static ref SHUFFLING_CACHE_MISSES: Result<IntCounter> =
         try_create_int_counter("beacon_shuffling_cache_misses_total", "Count of times shuffling cache fulfils request");
 
+    /*
+     * Early attester cache
+     */
+    pub static ref BEACON_EARLY_ATTESTER_CACHE_HITS: Result<IntCounter> = try_create_int_counter(
+        "beacon_early_attester_cache_hits",
+        "Count of times the early attester cache returns an attestation"
+    );
+
     /*
      * Attestation Production
      */