release-24.1: kv: shallow copy BatchRequest on mutate in tryBumpBatchTimestamp

[blathers-crl]

Backport 1/1 commits from #124634 on behalf of @lyang24.

Fixes #125126.

/cc @cockroachdb/release

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This avoids a data race on tryBumpBatchTimestamp, which was fallout from the new logging introduced in ba13697.

Fixes: #124553
Fixes: #126081

Release note: None

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Release justification: fixes data race.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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[cockroach-teamcity]

This change is 

[yuzefovich]

Should we merge this @arulajmani @nvanbenschoten? I think this would fix #126081.

[arulajmani]

IIRC @nvanbenschoten wanted to give this some bake time before we merge the backport; I'll defer to him on pressing merge.