Thread safety. Remove Atomic gem dependency.

Hi,
I removed class accessors for `Redis::Client`, because while `Atomic` guarantees atomic writes against that singleton values, if Rails doesn't have `Rack::Lock` activated it causes problems with the read of that values.

This implementation looks more complex than the current one, but ideally, most of the stuff should be pulled up in `Peek::View` (ie. `after_request`). I also think that `subscribe` should be responsible of storing subscribers for the request cycle and clean them up.