Tree states to support per-slot state diffs (#4652)

* Support per slot state diffs

* Store HierarchyConfig on disk. Support storing hdiffs at per slot level.

* Revert HierachyConfig change for testing.

* Add validity check for the hierarchy config when opening the DB.

* Update HDiff tests.

* Fix `get_cold_state` panic when the diff for the slot isn't stored.

* Use slots instead of epochs for storing snapshots in freezer DB.

* Add snapshot buffer to `diff_buffer_cache` instead of loading it from db every time.

* Add `hierarchy-exponents` cli flag to beacon node.

* Add test for `StorageStrategy::ReplayFrom` and ignore a flaky test.

* Drop hierarchy_config in tests for more frequent snapshot and fix an issue where hdiff wasn't stored unless it's a epoch boundary slot.

beacon_node/store/src/errors.rs

@@ -41,7 +41,7 @@ pub enum Error {
     },
     MissingStateRoot(Slot),
     MissingState(Hash256),
-    MissingSnapshot(Epoch),
+    MissingSnapshot(Slot),
     MissingDiff(Epoch),
     NoBaseStateFound(Hash256),
     BlockReplayError(BlockReplayError),