tornado: Fix race condition on handler._request.

Commit 6fd1a558b7 (#21469) introduced an
await point where get_events_backend calls fetch_events in order to
switch threads.  This opened the possibility that, in the window
between the connect_handler call in fetch_events and the old location
of this assignment in get_events_backend, an event could arrive,
causing ClientDescriptor.add_event to crash on missing
handler._request.  Fix this by assigning handler._request earlier.

Signed-off-by: Anders Kaseorg <anders@zulip.com>
(cherry picked from commit e112b619cc)

zerver/tornado/views.py

@@ -125,6 +125,7 @@ def get_events_backend(
     assert tornado_handler is not None
     handler = tornado_handler()
     assert handler is not None
+    handler._request = request
 
     if user_client is None:
         valid_user_client = RequestNotes.get_notes(request).client
@@ -173,7 +174,6 @@ def get_events_backend(
         # Mark this response with .asynchronous; this will result in
         # Tornado discarding the response and instead long-polling the
         # request.  See zulip_finish for more design details.
-        handler._request = request
         response = json_success(request)
         response.asynchronous = True
         return response