Block proposal optimisations (#8156)

Closes:

- https://github.com/sigp/lighthouse/issues/4412

This should reduce Lighthouse's block proposal times on Holesky and prevent us getting reorged.


  - [x] Allow the head state to be advanced further than 1 slot. This lets us avoid epoch processing on hot paths including block production, by having new epoch boundaries pre-computed and available in the state cache.
- [x] Use the finalized state to prune the op pool. We were previously using the head state and trying to infer slashing/exit relevance based on `exit_epoch`. However some exit epochs are far in the future, despite occurring recently.


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

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -5233,16 +5233,20 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             None
         };
 
+        let slashings_and_exits_span = debug_span!("get_slashings_and_exits").entered();
         let (mut proposer_slashings, mut attester_slashings, mut voluntary_exits) =
             self.op_pool.get_slashings_and_exits(&state, &self.spec);
+        drop(slashings_and_exits_span);
 
         let eth1_data = state.eth1_data().clone();
 
         let deposits = vec![];
 
+        let bls_changes_span = debug_span!("get_bls_to_execution_changes").entered();
         let bls_to_execution_changes = self
             .op_pool
             .get_bls_to_execution_changes(&state, &self.spec);
+        drop(bls_changes_span);
 
         // Iterate through the naive aggregation pool and ensure all the attestations from there
         // are included in the operation pool.