Fix HTTP repl response to use minimum token (#16578)

changelog.d/16578.bugfix

@@ -0,0 +1 @@
+Fix a long-standing, exceedingly rare edge case where the first event persisted by a new event persister worker might not be sent down `/sync`.

synapse/replication/http/_base.py

@@ -433,7 +433,7 @@ class ReplicationEndpoint(metaclass=abc.ABCMeta):
 
         if self.WAIT_FOR_STREAMS:
             response[_STREAM_POSITION_KEY] = {
-                stream.NAME: stream.current_token(self._instance_name)
+                stream.NAME: stream.minimal_local_current_token()
                 for stream in self._streams
             }