feat(pageserver): initial code sketch & test case for combined gc+compaction at gc_horizon

[skyzh]

Problem

A demo for a building block for compaction. The GC-compaction operation iterates all layers below/intersect with the GC horizon, and do a full layer rewrite of all of them. The end result will be image layer covering the full keyspace at GC-horizon, and a bunch of delta layers above the GC-horizon. This helps us collect the garbages of the test_gc_feedback test case to reduce space amplification.

This operation can be manually triggered using an HTTP API or be triggered based on some metrics. Actual method TBD.

The test is very basic and it's very likely that most part of the algorithm will be rewritten. I would like to get this merged so that I can have a basic skeleton for the algorithm and then make incremental changes.

Summary of changes

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]

3198 tests run: 3056 passed, 0 failed, 142 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_pageserver_restarts_under_worload: release

Code coverage* (full report)

• functions: 31.6% (6633 of 20979 functions)
• lines: 48.6% (51461 of 105800 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
90edb45 at 2024-06-11T14:23:48.082Z :recycle:}

[problame]

        // TODO: we should test a real delta record here, which requires us to add a variant of NeonWalRecord for testing purpose.

At the Istanbul offsite, Vlad proposed to add such a record type (guarded by #[cfg(test)].).

I think your next PR should be that.
Extend test_simple_bottom_most_compaction so we exercise the redo code path.

[problame]

More appropriate PR title (I don't think we should talk about levels, really):

initial code sketch & test case for combined gc+compaction at gc_horizon