queue: Only set QOS on a newly-opened channel, once.

As written, the QOS parameters are (re)set every time ensure_queue is
called, which is every time a message is enqueued. This is wasteful --
particularly QOS parameters only apply for consumers, and setting them
takes a RTT to the server.

Switch to only setting the QOS once, when a connection
is (re)established.  In profiling, this reduces the time to call
`queue_json_publish("noop", {})` from 878µs to 150µs.

zerver/lib/queue.py

@@ -160,9 +160,10 @@ class SimpleQueueClient(QueueClient[BlockingChannel]):
         the callback with no arguments."""
         if self.connection is None or not self.connection.is_open:
             self._connect()
-
             assert self.channel is not None
             self.channel.basic_qos(prefetch_count=self.prefetch)
+        else:
+            assert self.channel is not None
 
         if queue_name not in self.queues:
             self.channel.queue_declare(queue=queue_name, durable=True)