Batch subscription should be stoppable despite of pending message retries

[faderskd]

If you have a batch subscription sending messages to the receiver returning 5xx status codes, the messages are retried until TTL is passed. Sending with retries blocks BatchConsumer main loop, thus blocking processing consumer signals. If the subscription was stopped at the moment of retries, it will wait until retries are exhausted. This is not a desired behaviour. The subscription should be stopped immediately even if the are pending retries.