Refactor refactored token bucket API to be more generic

The legacy/trac#25766 (moved) work makes us have a clean token bucket API. But that API isn't reusable yet. Let's fix it so we can use it for all the other places in our code that we want a token bucket.

The two main things to do here are:

• decouple "read bucket" from "write bucket".
• decouple "parameters" from "bucket value".
• make time generic.