Fix bug where state_group tables got corrupted

This is due to the fact that we prefilled caches using txn.call_after,
which always gets called including on error.

We fix this by making txn.call_after only fire when a transaction
completes successfully, which is what we want most of the time anyway.

synapse/storage/__init__.py

@@ -225,7 +225,8 @@ class DataStore(RoomMemberStore, RoomStore,
             db_conn.cursor(),
             name="_find_stream_orderings_for_times_txn",
             database_engine=self.database_engine,
-            after_callbacks=[]
+            after_callbacks=[],
+            final_callbacks=[],
         )
         self._find_stream_orderings_for_times_txn(cur)
         cur.close()