Remove redundant get_current_events_token (#11643)

* Push `get_room_{min,max_stream_ordering}` into StreamStore

Both implementations of this are identical, so we may as well push it down and
get rid of the abstract base class nonsense.

* Remove redundant `StreamStore` class

This is empty now

* Remove redundant `get_current_events_token`

This was an exact duplicate of `get_room_max_stream_ordering`, so let's get rid
of it.

* newsfile

synapse/handlers/federation_event.py

@@ -1838,7 +1838,7 @@ class FederationEventHandler:
             The stream ID after which all events have been persisted.
         """
         if not event_and_contexts:
-            return self._store.get_current_events_token()
+            return self._store.get_room_max_stream_ordering()
 
         instance = self._config.worker.events_shard_config.get_instance(room_id)
         if instance != self._instance_name: