cache: refactor PeekLatest/PeekOldest

[apullo777]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

@serathius

[k8s-ci-robot]

Hi @apullo777. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: apullo777, MadhavJivrajani, serathius

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