Implement new replace_state and changed prev_state

`prev_state` is now a list of previous state ids, similiar to
prev_events. `replace_state` now points to what we think was replaced.

synapse/handlers/directory.py

@@ -147,10 +147,7 @@ class DirectoryHandler(BaseHandler):
             content={"aliases": aliases},
         )
 
-        snapshot = yield self.store.snapshot_room(
-            room_id=room_id,
-            user_id=user_id,
-        )
+        snapshot = yield self.store.snapshot_room(event)
 
         yield self._on_new_room_event(
             event, snapshot, extra_users=[user_id], suppress_auth=True