More proposer shuffling cleanup (#8130)

Addressing more review comments from:

- https://github.com/sigp/lighthouse/pull/8101

I've also tweaked a few more things that I think are minor bugs.


  - Instrument `ensure_state_can_determine_proposers_for_epoch`
- Fix `block_root` usage in `compute_proposer_duties_from_head`. This was a regression introduced in 8101 😬 .
- Update the `state_advance_timer` to prime the next-epoch proposer cache post-Fulu.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

beacon_node/beacon_chain/src/block_verification.rs

@@ -950,8 +950,6 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
         let proposer_shuffling_decision_block =
             parent_block.proposer_shuffling_root_for_child_block(block_epoch, &chain.spec);
 
-        // We assign to a variable instead of using `if let Some` directly to ensure we drop the
-        // write lock before trying to acquire it again in the `else` clause.
         let block_slot = block.slot();
         let mut opt_parent = None;
         let proposer = chain.with_proposer_cache::<_, BlockError>(