Move compaction code to compaction.rs

[arpad-m]

Moves some of the (legacy) compaction code to compaction.rs. No functional changes, just moves of code.

Before, compaction.rs was only for the new tiered compaction mechanism, now it's for both the old and new mechanisms.

Part of #6768

[github-actions]

2490 tests run: 2368 passed, 0 failed, 122 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 28.8% (6982 of 24255 functions)
• lines: 47.4% (42878 of 90535 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
2a58378 at 2024-03-05T17:30:38.415Z :recycle:}