Top-up pubkey cache on startup

[eserilev]

Issue Addressed

This is a workaround for #7216

Proposed Changes

In the case of gaps between the in-memory pub key cache and its on-disk representation, use the head state on startup to "top-up" the cache/db w/ any missing validators

[michaelsproul]

I don't think we wanna do this with the genesis state, we need the head state (or maybe the finalized state, but the head would be better)

[michaelsproul]

Here:

lighthouse/beacon_node/beacon_chain/src/builder.rs

Lines 817 to 820 in 491091f

	let (_head_state_root, head_state) = store
	.get_advanced_hot_state(head_block_root, current_slot, head_block.state_root())
	.map_err(|e| descriptive_db_error("head state", &e))?
	.ok_or("Head state not found in store")?;

[michaelsproul]

We'd do the top up in the Ok case here:

lighthouse/beacon_node/beacon_chain/src/builder.rs

Lines 861 to 864 in 491091f

	let validator_pubkey_cache = self.validator_pubkey_cache.map(Ok).unwrap_or_else(|| {
	ValidatorPubkeyCache::new(&head_snapshot.beacon_state, store.clone())
	.map_err(|e| format!("Unable to init validator pubkey cache: {:?}", e))
	})?;

[michaelsproul]

LGTM!