pageserver: add separate semaphore for L0 compaction

[erikgrinaker]

Problem

L0 compaction frequently gets starved out by other background tasks and image/GC compaction. L0 compaction must be responsive to keep read amplification under control.

Touches #10694.
Resolves #10689.

Summary of changes

Use a separate semaphore for the L0-only compaction pass.

• Add a CONCURRENT_L0_COMPACTION_TASKS semaphore and BackgroundLoopKind::L0Compaction.
• Add a setting compaction_l0_semaphore (default off via compaction_l0_first).
• Use the L0 semaphore when doing an OnlyL0Compaction pass.
• Use the background semaphore when doing a regular compaction pass (which includes an initial L0 pass).
• While waiting for the background semaphore, yield for L0 compaction if triggered.
• Add CompactFlags::NoYield to disable L0 yielding, and set it for the HTTP API route.
• Remove the old use_compaction_semaphore setting and compaction-scoped semaphore.
• Remove the warning when waiting for a semaphore; it's noisy and we have metrics.

[github-actions]

7458 tests run: 7078 passed, 0 failed, 380 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_scrubber_physical_gc_ancestors[None]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 33.2% (8581 of 25870 functions)
• lines: 49.1% (72317 of 147387 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
b01c54b at 2025-02-12T14:17:31.827Z :recycle:}

[erikgrinaker]

I ran this through CI enabled by default, which passed. Switched it back to disabled by default for now.