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/block_packing_efficiency.rs

@@ -285,8 +285,10 @@ pub fn get_block_packing_efficiency<T: BeaconChainTypes>(
     // Load state for block replay.
     let starting_state_root = first_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 starting_state = chain
-        .get_state(&starting_state_root, Some(prior_slot))
+        .get_state(&starting_state_root, Some(prior_slot), true)
         .and_then(|maybe_state| {
             maybe_state.ok_or(BeaconChainError::MissingBeaconState(starting_state_root))
         })