refactor(pageserver): move queue logic to compaction.rs

[skyzh]

Problem

close #10031, part of #9114

Summary of changes

Move the compaction job generation to compaction.rs, thus making the code more readable and debuggable. We now also return running job through the get compaction job API, versus before we only return scheduled jobs.

[github-actions]

7282 tests run: 6917 passed, 0 failed, 365 skipped (full report)

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Code coverage* (full report)

• functions: 32.6% (8041 of 24632 functions)
• lines: 47.7% (66823 of 140018 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
07e6936 at 2025-01-10T20:16:37.370Z :recycle:}