[persist] Don't assume old parts are totally consolidated

[bkirwi]

Motivation

Support for #21615.

In particular, this is necessary if we want to assume that the output of the Consolidator is indeed consolidated.

Tips for reviewer

To my knowledge the last important bug was fixed in df46c74, and 0.67.0 is the first release that includes it.

I think this means it's safe to assert on ordering in the consolidator now, but I'll run all the upgrade tests to make sure.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• 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).
• This PR includes the following user-facing behavior changes:

[danhhz]

lgtm!

would love to see the de-dent happen. deduping WriterKey parsing logic i'll leave to your judgement

[bkirwi]

Landed both - thanks for the review!