## 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`.
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