Avoid looking up pre-finalization blocks (#2909)

## Issue Addressed

This PR fixes the unnecessary `WARN Single block lookup failed` messages described here:

https://github.com/sigp/lighthouse/pull/2866#issuecomment-1008442640

## Proposed Changes

Add a new cache to the `BeaconChain` that tracks the block roots of blocks from before finalization. These could be blocks from the canonical chain (which might need to be read from disk), or old pre-finalization blocks that have been forked out.

The cache also stores a set of block roots for in-progress single block lookups, which duplicates some of the information from sync's `single_block_lookups` hashmap:

a836e180f9/beacon_node/network/src/sync/manager.rs (L192-L196)

On a live node you can confirm that the cache is working by grepping logs for the message: `Rejected attestation to finalized block`.

beacon_node/network/src/beacon_processor/worker/gossip_methods.rs

@@ -1701,6 +1701,26 @@ impl<T: BeaconChainTypes> Worker<T> {
                     "attn_too_many_skipped_slots",
                 );
             }
+            AttnError::HeadBlockFinalized { beacon_block_root } => {
+                debug!(
+                    self.log,
+                    "Rejected attestation to finalized block";
+                    "block_root" => ?beacon_block_root,
+                    "attestation_slot" => failed_att.attestation().data.slot,
+                );
+
+                // We have to reject the message as it isn't a descendant of the finalized
+                // checkpoint.
+                self.propagate_validation_result(message_id, peer_id, MessageAcceptance::Reject);
+
+                // The peer that sent us this could be a lagger, or a spammer, or this failure could
+                // be due to us processing attestations extremely slowly. Don't be too harsh.
+                self.gossip_penalize_peer(
+                    peer_id,
+                    PeerAction::HighToleranceError,
+                    "attn_to_finalized_block",
+                );
+            }
             AttnError::BeaconChainError(BeaconChainError::DBError(Error::HotColdDBError(
                 HotColdDBError::AttestationStateIsFinalized { .. },
             ))) => {