Use a redis set to store queue names, instead of relying on the redis "keys" method

Curious to your thoughts on this. I'm using an appraach similar to the way you store worker info in redis as a Set.

The keys("prefix*") call has notoriously bad performance on large databases. In took down our servers today while trying to run rq-dashboard. This would mitigate that, however there is currently no way to remove an old queue name from the set. I'm not sure if this is a huge deal or not. But it would be trivial to add a deregister queue method if needed.