message_send: Update do_send_messages codepath to send event on commit.

Earlier, we were using 'send_event' & 'queue_json_publish' in
'do_send_messages' which can lead to a situation where we enqueue
events but the transaction fails at a later stage.

Events should not be sent until we know we're not rolling back.

zerver/tests/test_message_send.py

@@ -1769,6 +1769,7 @@ class StreamMessagesTest(ZulipTestCase):
                 user,
                 stream_name,
                 content=content,
+                skip_capture_on_commit_callbacks=True,
             )
         users = events[0]["users"]
         user_ids = {u["id"] for u in users}