fix(pageserver): skip repartition if we need L0 compaction

[skyzh]

Problem

Repartition is slow, but it's only used in image layer creation. We can skip it if we have a lot of L0 layers to ingest.

Summary of changes

If L0 compaction is not complete, do not repartition and do not create image layers.

[github-actions]

7414 tests run: 7062 passed, 0 failed, 352 skipped (full report)

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Flaky tests (7)

Postgres 17

• test_pgdata_import_smoke[None-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: debug-x86-64-without-lfc, release-arm64-with-lfc, release-arm64-without-lfc
• test_pgdata_import_smoke[8-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: debug-x86-64-without-lfc, release-arm64-with-lfc, release-arm64-without-lfc

Postgres 16

• test_pageserver_gc_compaction_idempotent[after_restart]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 33.3% (8495 of 25482 functions)
• lines: 49.1% (71413 of 145486 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
eeebfd1 at 2025-01-29T21:42:09.235Z :recycle:}