fix #653, overuse of Redis KEYS in mirror

This is a bit speculative, I think it should still do the right thing if the lists expire while they're in the middle of being processed (which was also possible with the previous code, of course). @jessereynolds, I'll need your review and signoff before merging, as you may see problems I didn't with this approach.
We'll need to come up with a way of removing members from known_events_archive_keys when corresponding archive keys are expired? The expiry is set here: https://github.com/flapjack/flapjack/blob/653_improve_mirror_performance/lib/flapjack/data/event.rb#L111