release-25.3: kvnemesis: add another buffered-writes variant

[blathers-crl]

Backport 1/1 commits from #148899 on behalf of @stevendanna.

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Until we are confident in the lock durability upgrading code, let's test in its absence. This will also ensure we are losing locks at some rate and stress the new lock-loss detection.

Epic: none
Release note: None

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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[cockroach-teamcity]

This change is