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/state_advance_timer.rs

@@ -321,6 +321,12 @@ fn advance_head<T: BeaconChainTypes>(
         );
     }
 
+    // Apply the state to the attester cache, if the cache deems it interesting.
+    beacon_chain
+        .attester_cache
+        .maybe_cache_state(&state, head_root, &beacon_chain.spec)
+        .map_err(BeaconChainError::from)?;
+
     let final_slot = state.slot();
 
     // Insert the advanced state back into the snapshot cache.