feat(pageserver): split delta writer automatically determines key range

[skyzh]

Problem

close #8838

Summary of changes

This patch modifies the split delta layer writer to avoid taking start_key and end_key when creating/finishing the layer writer. The start_key for the delta layers will be the first key provided to the layer writer, and the end_key would be the last_key.next(). This simplifies the delta layer writer API.

On that, the layer key hack is removed. Image layers now use the full key range, and delta layers use the first/last key provided by the user.

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]

3883 tests run: 3768 passed, 1 failed, 114 skipped (full report)

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Failures on Postgres 14

• test_storage_controller_heartbeats[failure4]: release-x86-64

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_storage_controller_heartbeats[release-pg14-failure4]"

Flaky tests (2)

Postgres 15

• test_scrubber_physical_gc[4]: release-x86-64

Postgres 14

• test_pageserver_chaos[None]: release-arm64

Code coverage* (full report)

• functions: 31.7% (7343 of 23156 functions)
• lines: 49.9% (59528 of 119196 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
a2138c2 at 2024-09-09T20:44:28.340Z :recycle:}

[problame]

The NON_L0_MAX can be removed in this PR

neon/libs/pageserver_api/src/key.rs

Line 240 in 0f65684

pub const NON_L0_MAX: Key = Key {

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca