Ensure that there are no subscribers if there is no Redis connection

I experienced issues if my Redis server was restarted: the list of subscribers didn't change while there was no subscription to a channel in Redis.

By resetting the subscribers if there is no connection we ensure that there will be a subscription to a channel in Redis (rather than piling up subscribers that will never receive a message).

(Not all tests were passing when I ran the tests before I applied this so I don't think this will break anything)
I think this won't solve the issue.
I think we need to add a "reconnect" event handler, sending "subscribe/psubscribe" messages for each handled channel.