storage/kafka: Shrink buffers in sink_collection operator

[ParkMyCar]

In the Kafka sink_collection operator we re-use two Vecs to buffer messages, but never shrink them. So it's possible for a large update to blow up the size of these Vecs and then we're stuck holding the heap allocation for a while.

Motivation

Possibly fix an issue we've been seeing.

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.

[petrosagg]

Thank you!