Avoid Redis KEYS in remove_delayed_selection

Do not use Redis KEYS command in Resque.remove_delayed_selection,
iterate through all timestamps instead.

Note that it can takes a lot of time to execute and also you might
consider what happens if you try to delete all delayed matching a given
selection criteria, while someone else is going to add new jobs.

CREDITS:
- @yaauie for the first draft in #411
- @xyzjace for following up in #415
- @potomak to help in debugging issues with the provided PR