refactor: make cachekv store thread-safe again (backport #14378)

[mergify]

This is an automatic backport of pull request #14378 done by Mergify.
Cherry-pick of f1ee974 has failed:

On branch mergify/bp/release/v0.47.x/pr-14378
Your branch is up to date with 'origin/release/v0.47.x'.

You are currently cherry-picking commit f1ee974ec.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   store/cachekv/internal/btree_test.go
	modified:   store/cachekv/store.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   store/cachekv/internal/btree.go
	both modified:   store/cachekv/internal/memiterator.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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