zmirror: Attempt to receive zephyrs as we send them.

Sometimes the very first message we send isn't received by
python-zephyr; this could be because of our growing a 17-message queue
of zephyrs to service, so let's not do that.

(imported from commit 281bf1807442b6335b05c803b1a47e0a162bef4e)

bots/check-mirroring

@@ -230,6 +230,7 @@ for key, (stream, test) in zhkeys.items():
         else:
             logging.warning("Replaced key %s with %s due to Zephyr server failure." %
                             (key, new_key))
+    receive_zephyrs()
 
 receive_zephyrs()
 logger.info("Sent Zephyr messages!")