[persist] Monoid bound for the diff type in Persist

[bkirwi]

Motivation

Right now a diff sum of None sometimes means "zero", and sometimes means "unknown". This makes it harder to write assertions on the diffs... replacing a 0-sum batch with a 7-sum batch is clearly bad, but replacing an unknown-sum batch with a 7-sum batch might be fine.

Tips for reviewer

This adds a stronger requirement for all Persist usage. This would probably not be worth it if we thought we were realistically going to use Persist with a non-monoid diff type... but that seems very unlikely, so this seems worth doing to avoid any additional risk of errors.

First commit has the search-and-replace; second commit has a small amount of code that takes advantage of the new bound.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.