only retry if the broadcast message task is in sending

previously we would retry if the task was queued up for retry but the
status is in "received-ack" or "received-err". We don't expect that a
task will be retried after getting this status, but if there are
duplicate tasks that could happen. Lets plan for the worst by saying
"only process a retry if the task is currently in sending".

this way, if a duplicate task is on retry and the first task goes
through succesfully, the duplicate task will give up.

app/__init__.py

@@ -381,7 +381,7 @@ def setup_sqlalchemy_events(app):
                         'host': current_app.config['NOTIFY_APP_NAME'],  # worker name
                         'url_rule': current_task.name,  # task name
                     }
-                # anything else. migrations possibly.
+                # anything else. migrations possibly, or flask cli commands.
                 else:
                     current_app.logger.warning('Checked out sqlalchemy connection from outside of request/task')
                     connection_record.info['request_data'] = {