Add AttesterCache for attestation production (#2478)

## Issue Addressed

- Resolves #2169

## Proposed Changes

Adds the `AttesterCache` to allow validators to produce attestations for older slots. Presently, some arbitrary restrictions can force validators to receive an error when attesting to a slot earlier than the present one. This can cause attestation misses when there is excessive load on the validator client or time sync issues between the VC and BN.

## Additional Info

NA

beacon_node/beacon_chain/src/builder.rs

@@ -547,6 +547,7 @@ where
             shuffling_cache: TimeoutRwLock::new(ShufflingCache::new()),
             beacon_proposer_cache: <_>::default(),
             validator_pubkey_cache: TimeoutRwLock::new(validator_pubkey_cache),
+            attester_cache: <_>::default(),
             disabled_forks: self.disabled_forks,
             shutdown_sender: self
                 .shutdown_sender
@@ -561,6 +562,16 @@ where
             .head()
             .map_err(|e| format!("Failed to get head: {:?}", e))?;
 
+        // Prime the attester cache with the head state.
+        beacon_chain
+            .attester_cache
+            .maybe_cache_state(
+                &head.beacon_state,
+                head.beacon_block_root,
+                &beacon_chain.spec,
+            )
+            .map_err(|e| format!("Failed to prime attester cache: {:?}", e))?;
+
         // Only perform the check if it was configured.
         if let Some(wss_checkpoint) = beacon_chain.config.weak_subjectivity_checkpoint {
             if let Err(e) = beacon_chain.verify_weak_subjectivity_checkpoint(