Support old finalized checkpoints in assertPeerRelevance

From https://github.com/ChainSafe/lodestar/pull/3405

TODO: If the slot is very old, go to the historical blocks DB and fetch the block with less or equal `slot`. Note that our db schema will have to be updated to persist the block root to prevent re-hashing. For now peers will be accepted, since it's better than throwing an error on `getBlockRootAtSlot()`

https://github.com/ChainSafe/lodestar/blob/fbeabc6acb9bd9b018c10f58ee0d1291843371b0/packages/lodestar/src/network/peers/utils/assertPeerRelevance.ts#L97-L99

Start in packages/lodestar/src/network/peers/utils/assertPeerRelevance.ts at the referenced getBlockRootAtSlot() call. Trace how finalized checkpoints are resolved and how the historical blocks DB stores blocks. Done means an old slot uses the block at or before that slot, with its block root persisted to avoid re-hashing.

Written by the indexing model from the issue text.