Ensure RedisBackend Listener Restarts on Resubscription

[lexthink]

This pull request addresses a specific issue in the RedisBackend class where if all subscribers to a channel were unsubscribed, the listener task would complete and not restart upon new subscriptions. This led to a scenario where messages could be missed if new subscriptions were added after all others were removed.

[alex-oleshkevich]

We have the same concern for postgres backend. I am not sure if we want to have automatic reconnects in the backend. This can cause another issues like extra configuration (how many retries allowed? What is the back off value? What to do if Redis is permanently down?).

So I think that the best approach will be just to ignore disconnects and exit the context manager or raise the exception. Developers then can decide the proper solution for their needs.