Advance replication streams even if nothing is listening

Otherwise the streams don't advance and steadily fall behind, so when a
worker does connect either a) they'll be streamed lots of old updates or
b) the connection will fail as the streams are too far behind.

synapse/replication/tcp/resource.py

@@ -124,7 +124,7 @@ class ReplicationStreamer(object):
             # Don't bother if nothing is listening. We still need to advance
             # the stream tokens otherwise they'll fall beihind forever
             for stream in self.streams:
-                stream.advance_current_token()
+                stream.discard_updates_and_advance()
             return
 
         # If we're in the process of checking for new updates, mark that fact