Rate limits cleanup (#19990)

* use monotonic clock in rate limiter

* rename minute -> slot in rate limiter, as the length is a parameter

* mock timer in rate limit test to make them deterministic

* remove unnecessary pytest class

* improve comment in rate_limit_numbers

* fix minor typos in rate_limits.py

* replace test_too_many_messages() with a parametrized test for (almost) all rate-limit cases

* review comments: give better names to test parameters

* review comments

* review comments

* review comments

* review comments

Author: arvidn