tornado: Close queue connection on reload.

Tornado reloads the app whenever there is a change in code. Due to this,
new connection is created to the client which also results in a new
channel. To avoid creating two channels for the queue in the RabbitMQ
broker we should close the old channel. Otherwise messages sent to the
queue will be distributed among these two channels in a round robin
scheme and we will end up losing one message since one of the channels
doesn't have an active consumer.

This commit closes the connection to the queue whenever Tornado reloads
the application using add_reload_hook().

Fixes #5824.

zerver/tornado/application.py

@@ -9,6 +9,7 @@ from zerver.tornado.handlers import AsyncDjangoHandler
 from zerver.tornado.socket import get_sockjs_router
 from zerver.lib.queue import get_queue_client
 
+import tornado.autoreload
 import tornado.web
 
 def setup_tornado_rabbitmq():
@@ -17,6 +18,7 @@ def setup_tornado_rabbitmq():
     if settings.USING_RABBITMQ:
         queue_client = get_queue_client()
         atexit.register(lambda: queue_client.close())
+        tornado.autoreload.add_reload_hook(lambda: queue_client.close())  # type: ignore # TODO: Fix missing tornado.autoreload stub
 
 def create_tornado_application():
     # type: () -> tornado.web.Application