I need to filter requests on a period I need to get dynamically out of
information I have in the requests. Currently, I can work out the limit,
as it can be a `Proc`, however I can't do that with the period.
This PR adds support for that. Tried to do it in a way that doesn't
brake backwards compatibility, as periods are coerced to numbers during
`Rack::Throttle` initialization.
Throttles use a cache key with a timestamp (Time.now.to_i/period), so a
new cache key is used for each period.
No longer set an explicit expiry on each cache key (though it may
inherit a default expiry from the cache store).
Also, set env['rack.attack.throttle_data'] with info about incremented
(but not necessarily exceeded) throttles.
