The old name `push_notify_user_ids` was misleading, because
it does not contain user ids which should be notified for
the current message, but rather user ids who have the online
push notifications setting enabled.
When the Tornado server is restarted during an upgrade, if
server has old events with the `push_notify_user_ids` fields,
the server will throw error after this rename. Hence, we need
to explicitly handle such cases while processing the event.
