Merge pull request #1016 from matrix-org/erikj/short_circuit_cache

Don't update caches replication stream if tokens haven't advanced

synapse/storage/_base.py

@@ -894,6 +894,9 @@ class SQLBaseStore(object):
             )
 
     def get_all_updated_caches(self, last_id, current_id, limit):
+        if last_id == current_id:
+            return defer.succeed([])
+
         def get_all_updated_caches_txn(txn):
             # We purposefully don't bound by the current token, as we want to
             # send across cache invalidations as quickly as possible. Cache