KAFKA-5998: fix checkpointableOffsets handling

[vvcephei]

• each task should write only the checkpoint offsets for changelog/partitions that it owns
• check upon loading checkpointableOffsets and also immediately upon writing checkpointableOffsets that the task actually owns the partitions in question. This will prevent checkpoint file corruption in the future, and also help people clear out their corrupted checkpoint files so that they can be rebuild correctly.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[vvcephei]

This PR is targeted at 2.2 so that @pkleindl can verify the fix. I'll move it to trunk later.

I still need to write/update the integration tests for this condition (and actually run them to make sure I didn't break anything else).

[vvcephei]

I've rebased on trunk and opened #7030 for actual review and merging. This PR will just get closed once @pkleindl confirms the fix.

[vvcephei]

Closed in favor of #7030