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/http_api/src/proposer_duties.rs

@@ -103,14 +103,6 @@ fn try_proposer_duties_from_cache<T: BeaconChainTypes>(
     let head_block = &head.snapshot.beacon_block;
     let head_block_root = head.head_block_root();
     let head_epoch = head_block.slot().epoch(T::EthSpec::slots_per_epoch());
-    let head_decision_root = head
-        .snapshot
-        .beacon_state
-        .proposer_shuffling_decision_root(head_block_root, &chain.spec)
-        .map_err(warp_utils::reject::beacon_state_error)?;
-    let execution_optimistic = chain
-        .is_optimistic_or_invalid_head_block(head_block)
-        .map_err(warp_utils::reject::unhandled_error)?;
 
     // This code path can't handle requests for past epochs.
     if head_epoch > request_epoch {
@@ -119,6 +111,15 @@ fn try_proposer_duties_from_cache<T: BeaconChainTypes>(
         )));
     }
 
+    let head_decision_root = head
+        .snapshot
+        .beacon_state
+        .proposer_shuffling_decision_root_at_epoch(request_epoch, head_block_root, &chain.spec)
+        .map_err(warp_utils::reject::beacon_state_error)?;
+    let execution_optimistic = chain
+        .is_optimistic_or_invalid_head_block(head_block)
+        .map_err(warp_utils::reject::unhandled_error)?;
+
     chain
         .beacon_proposer_cache
         .lock()