tornado: Move SIGTERM shutdown handler into a callback.

A SIGTERM can show up at any point in the ioloop, even in places which
are not prepared to handle it.  This results in the process ignoring
the `sys.exit` which the SIGTERM handler calls, with an uncaught
SystemExit exception:

```
2021-11-09 15:37:49.368 ERR  [tornado.application:9803] Uncaught exception
Traceback (most recent call last):
  File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/http1connection.py", line 238, in _read_message
    delegate.finish()
  File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/httpserver.py", line 314, in finish
    self.delegate.finish()
  File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/routing.py", line 251, in finish
    self.delegate.finish()
  File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/web.py", line 2097, in finish
    self.execute()
  File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/web.py", line 2130, in execute
    **self.path_kwargs)
  File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/gen.py", line 307, in wrapper
    yielded = next(result)
  File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/tornado/web.py", line 1510, in _execute
    result = method(*self.path_args, **self.path_kwargs)
  File "/home/zulip/deployments/2021-11-08-05-10-23/zerver/tornado/handlers.py", line 150, in get
    request = self.convert_tornado_request_to_django_request()
  File "/home/zulip/deployments/2021-11-08-05-10-23/zerver/tornado/handlers.py", line 113, in convert_tornado_request_to_django_request
    request = WSGIRequest(environ)
  File "/home/zulip/deployments/2021-11-08-05-10-23/zulip-py3-venv/lib/python3.6/site-packages/django/core/handlers/wsgi.py", line 66, in __init__
    script_name = get_script_name(environ)
  File "/home/zulip/deployments/2021-11-08-05-10-23/zerver/tornado/event_queue.py", line 611, in <lambda>
    signal.signal(signal.SIGTERM, lambda signum, stack: sys.exit(1))
SystemExit: 1
```

Supervisor then terminates the process with a SIGKILL, which results
in dropping data held in the tornado process, as it does not dump its
queue.

The only command which is safe to run in the signal handler is
`ioloop.add_callback_from_signal`, which schedules the callback to run
during the course of the normal ioloop.  This callbacks does an
orderly shutdown of the server and the ioloop before exiting.

zerver/management/commands/runtornado.py

@@ -104,7 +104,7 @@ class Command(BaseCommand):
                 from zerver.tornado.ioloop_logging import logging_data
 
                 logging_data["port"] = str(port)
-                setup_event_queue(port)
+                setup_event_queue(http_server, port)
                 add_client_gc_hook(missedmessage_hook)
                 setup_tornado_rabbitmq()