feat(governor): simpler rate limiting

While preserving the leaky bucket algorithm, the throttling aspect has
been greatly simplified. The new variant places the burden of
responding to backpressure on connected clients.

New configuration:
- throttle_milliseconds

Removed configuration:
- sleep_factor

Changed configuration:
- soft_limit: gone
- hard_limit -> limit

Changed names:
- _create_calc_sleep -> _create_limiter
- calc_sleep -> check_limit