[persist] Don't discard parts that should be deleted

[bkirwi]

Calling mark_consumed drops all the parts in the batch, so the rest of this method's logic didn't do anything.

Motivation

Bug! First reported in Slack.

Tips for reviewer

The slack thread also suggests that compare_and_append_batch should take ownership of the data. I've taken a look at this, but it's going to be a more substantial change... still seems worth doing, but better as a followup.

This also includes a simple regression test.

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.

[bkirwi]

Nightlies: https://buildkite.com/materialize/nightly/builds/10779