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/room.py

@@ -122,10 +122,7 @@ class RoomCreationHandler(BaseHandler):
 
         @defer.inlineCallbacks
         def handle_event(event):
-            snapshot = yield self.store.snapshot_room(
-                room_id=room_id,
-                user_id=user_id,
-            )
+            snapshot = yield self.store.snapshot_room(event)
 
             logger.debug("Event: %s", event)
 
@@ -364,10 +361,8 @@ class RoomMemberHandler(BaseHandler):
         """
         target_user_id = event.state_key
 
-        snapshot = yield self.store.snapshot_room(
-            event.room_id, event.user_id,
-            RoomMemberEvent.TYPE, target_user_id
-        )
+        snapshot = yield self.store.snapshot_room(event)
+
         ## TODO(markjh): get prev state from snapshot.
         prev_state = yield self.store.get_room_member(
             target_user_id, event.room_id
@@ -442,10 +437,7 @@ class RoomMemberHandler(BaseHandler):
             content=content,
         )
 
-        snapshot = yield self.store.snapshot_room(
-            room_id, joinee.to_string(), RoomMemberEvent.TYPE,
-            joinee.to_string()
-        )
+        snapshot = yield self.store.snapshot_room(new_event)
 
         yield self._do_join(new_event, snapshot, room_host=host, do_auth=True)