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

[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).

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• Start Time: 2024-04-08T06:46:25.344+0000

• Duration: 131 min 21 sec

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Total	31251

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @marc-gr? 🙏