State cache tweaks (#7095)

Backport of:

- https://github.com/sigp/lighthouse/pull/7067

For:

- https://github.com/sigp/lighthouse/issues/7039


  - Prevent writing to state cache when migrating the database
- Add `state-cache-headroom` flag to control pruning
- Prune old epoch boundary states ahead of mid-epoch states
- Never prune head block's state
- Avoid caching ancestor states unless they are on an epoch boundary
- Log when states enter/exit the cache

Co-authored-by: Eitan Seri-Levi <eserilev@ucsc.edu>

beacon_node/http_api/src/attestation_performance.rs

@@ -126,8 +126,11 @@ pub fn get_attestation_performance<T: BeaconChainTypes>(
 
     // Load state for block replay.
     let state_root = prior_block.state_root();
+
+    // This branch is reached from the HTTP API. We assume the user wants
+    // to cache states so that future calls are faster.
     let state = chain
-        .get_state(&state_root, Some(prior_slot))
+        .get_state(&state_root, Some(prior_slot), true)
         .and_then(|maybe_state| maybe_state.ok_or(BeaconChainError::MissingBeaconState(state_root)))
         .map_err(unhandled_error)?;