Begin by creating a lowest-level triple of the types needed to
implement a token bucket: a configuration, a timestamp, and the raw
bucket itself.
Note that for low-level buckets, the units of the timestamp and the
bucket itself are unspecified: each user can use a different type.
(This patch breaks check-spaces; a later patch will fix it)
This is a simple search-and-replace to rename the token bucket type
to indicate that it contains both a read and a write bucket, bundled
with their configuration. It's preliminary to refactoring the
bucket type.
This differs from our previous token bucket abstraction in a few
ways:
1) It is an abstraction, and not a collection of fields.
2) It is meant to be used with monotonic timestamps, which should
produce better results than calling gettimeofday over and over.
