Extend rate limiter to support whole operation execution time, not just the start

[adamw]

Currently, the rate limiter only takes into account the start of execution of an operation - and limits basing on this.

However, in some scenarios we should take into account the whole duration. E.g. if there's an HTTP request which we want to rate limit, the actual HTTP invocation might happen anytime between the start and end of an httpRequest.send(...) call.

The goal of this task is to design an API which would extend what's currently available, and support both start-time & whole-duration rate limits, using all three algorithms (fixed windows, sliding window, leaky bucket)