rabbitmq: Add on-close callback atomically in creating the connection.

Adding it afterward is inherently racy, and upstream's API is quite
reasonable for avoiding that -- just like we can pass an on-open
callback up front, we can do the same with the on-close callback.

This is a more thorough version of 4adf2d5c2 from back in 2013-04.

zerver/lib/queue.py

@@ -187,8 +187,8 @@ class TornadoQueueClient(SimpleQueueClient):
         self.connection = ExceptionFreeTornadoConnection(
             self._get_parameters(),
             on_open_callback = self._on_open,
+            on_close_callback = self._on_connection_closed,
         )
-        self.connection.add_on_close_callback(self._on_connection_closed)
 
     def _reconnect(self) -> None:
         self.connection = None