refactor(pageserver): refactor split writers using batch layer writer

[skyzh]

Problem

part of #9114, #8836, #8362

The split layer writer code can be used in a more general way: the caller puts unfinished writers into the batch layer writer and let batch layer writer to ensure the atomicity of the layer produces.

Summary of changes

• Add batch layer writer, which atomically finishes the layers. BatchLayerWriter::finish is simply a copy-paste from previous split layer writers.
• Refactor split writers to use the batch layer writer.
• The current split writer tests cover all code path of batch layer writer.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5238 tests run: 5024 passed, 0 failed, 214 skipped (full report)

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Code coverage* (full report)

• functions: 31.4% (7676 of 24436 functions)
• lines: 48.9% (60309 of 123413 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c5ff298 at 2024-10-23T19:55:45.027Z :recycle:}