Recompute keys in atomic incr with sliding window #69
This breaks the API because we change
Change the `WindowRateLimiter` to pass a callable to `incr_and_sum` so that during atomic increments the time windows can be recomputed in the event of a lock. This breaks the API because we change `incr_and_sum`'s `keys` arg to a callable. We keep backwards compatibility by checking if `keys` is callable and calling it on each use.