release-23.2: logstore: pool MVCCStats to avoid hot path allocs

[blathers-crl]

Backport 1/1 commits from #113742 on behalf of @pavelkalinnikov.

/cc @cockroachdb/release

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Before this commit, logstore.logAppend allocated MVCCStats on heap despite its lifetime being confined within this function's scope.

This commit switches to allocating MVCCStats on a sync.Pool. Since logAppend already has another pooled roachpb.Value with exactly the same lifetime, coalesce the allocation of both and put them in the same pool. This makes the cost of this change zero.

Microbenchmark results:

// before
BenchmarkLogStore_StoreEntries/bytes=1.0_KiB-24 837254   1270 ns/op  333 B/op  1 allocs/op
// after
BenchmarkLogStore_StoreEntries/bytes=1.0_KiB-24 1000000  1153 ns/op  157 B/op  0 allocs/op

Part of #111561
Epic: none
Release note: none

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Release justification: microbenchmark regression fix

[blathers-crl]

Thanks for opening a backport.

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[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