Allow using clock objects instead of hanging it off the type

[antifuchs]

This addresses #29:

• Rework rate-limiting states to take a Clock trait implementation.
• Tease apart the places where we to I::now() and use that clock object instead.

[antifuchs]

I think this is as tidy as it'll get now!

bors r+

[bors[bot]]

Build succeeded

• ci/circleci: ci_success