storage/kafka: notify reader on partition info change

[teskje]

When the number of partitions changed, we need to wake up the reader so he can assign itself to any new partitions it should consume.

Currently blocked by a race in source statistics: Slack.

Motivation

• This PR fixes a previously unreported bug.

See this Slack thread.

Tips for reviewer

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.