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/state_id.rs

@@ -189,8 +189,10 @@ impl StateId {
             _ => (self.root(chain)?, None),
         };
 
+        // 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, slot_opt)
+            .get_state(&state_root, slot_opt, true)
             .map_err(warp_utils::reject::unhandled_error)
             .and_then(|opt| {
                 opt.ok_or_else(|| {