fix(pageserver): make delta split layer writer finish atomic

[skyzh]

Problem

similar to #8841, we make the delta layer writer atomic when finishing the layers.

Summary of changes

• put_value not taking discard fn anymore
• finish decides what layers to keep

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]

5281 tests run: 5067 passed, 0 failed, 214 skipped (full report)

---

Flaky tests (2)

Postgres 17

• test_readonly_node_gc: debug-x86-64
• test_timeline_offloading[False]: release-arm64

Code coverage* (full report)

• functions: 31.4% (7653 of 24388 functions)
• lines: 48.9% (60198 of 123134 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
b4ae4c4 at 2024-10-22T22:17:10.210Z :recycle:}