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/beacon_chain/src/builder.rs

@@ -752,6 +752,7 @@ where
             shuffling_cache: TimeoutRwLock::new(ShufflingCache::new()),
             beacon_proposer_cache: <_>::default(),
             block_times_cache: <_>::default(),
+            pre_finalization_block_cache: <_>::default(),
             validator_pubkey_cache: TimeoutRwLock::new(validator_pubkey_cache),
             attester_cache: <_>::default(),
             early_attester_cache: <_>::default(),