kv: shallow copy {,Reverse}ScanResponse.{BatchResponses,ColBatches} when pipelining

[nvanbenschoten]

Fixes #121759.

This commit updates ScanResponse and ReverseScanResponse's ShallowCopy implementations to perform a shallow copy of the BatchResponses and ColBatches slices. These are both slices of data array slices which can be mutated by popBatch, even though the underlying data slices are immutable.

Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[nvanbenschoten]

These are both slices of data array slices which can be mutated by popBatch, even though the underlying data slices are immutable.

@yuzefovich could you confirm that the underlying data slices in {,Reverse}ScanResponse.BatchResponses are treated as immutable, even though the outer slice is mutated during query execution (by popBatch)?

[yuzefovich]

Yes, each []byte slice shouldn't be modified later - we only modify [][]byte to allow for faster GC of no longer needed []byte slices.

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @arulajmani)

[nvanbenschoten]

TFTR!

bors r+

[craig]

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