tornado: Support clearing a handler more than once.

4af00f61a8 claimed that `on_finish` and
`on_connection_close` were mutually exclusive.  In cases where a
`DELETE` is called on the queue while a longpoll is in progress, this
can cause _both_ to happen:

- The `DELETE` pushes a `cleanup_queue` event, which triggers
`finish_handler` to begin pushing out an empty event response to the
longpoll connection.

- In the midst of that, in an `await`, the longpoll connection drops,
and `on_connection_close` clears the handler.

- The `await` resumes, calls `finish`, and attempts to clear the
handler.

The easiest solution is to make `clear_handler_by_id` tolerant to
multiple attempts to clear it.  Since these processes run in parallel,
it means that parts may have a `handler_id` but `get_handler_by_id`
may error in attempting to look it up.  We have not observed this in
testing, and I cannot currently prove it is impossible.

zerver/tornado/handlers.py

@@ -37,6 +37,7 @@ def allocate_handler_id(handler: "AsyncDjangoHandler") -> int:
 
 
 def clear_handler_by_id(handler_id: int) -> None:
+    if handler_id in handlers:
         del handlers[handler_id]