Prevent writing to state cache when migrating the database (#7067)

* add an update_cache flag to get_state to have more granular control over when we write to the cache

* State cache tweaks

- 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

---------

Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/beacon_chain/src/migrate.rs

@@ -356,7 +356,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> BackgroundMigrator<E, Ho
         let finalized_state_root = notif.finalized_state_root;
         let finalized_block_root = notif.finalized_checkpoint.root;
 
-        let finalized_state = match db.get_state(&finalized_state_root.into(), None) {
+        let finalized_state = match db.get_state(&finalized_state_root.into(), None, true) {
             Ok(Some(state)) => state,
             other => {
                 error!(