[libbeat] Cache processor docs and memory fixes. (backport #38561)

[mergify]

Proposed commit message

• Change cache processor documentation from write_period to write_interval.
• Fix expiries heap cleanup on partial file writes: popping the expiries heap completely on every partial write caused a panic on the following put operations at the time of expiry check.
• Fix expiries infinite growth when large TTLs and recurring keys are cached: when TTLs are large and keys are recurrent, we kept track of an arbitrary amount of expiry heap entries for the same key, causing memory to grow.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

---

This is an automatic backport of pull request #38561 done by [Mergify](https://mergify.com).

[botelastic]

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[elasticmachine]

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[cmacknz]

The test failures are due to a missing backport, which was omitted as there are no more planned 8.12 releases. Force merging this to enable testing of the fix here.