fix(pageserver): ensure initial image layers have correct key ranges

[skyzh]

Problem

Discovered during the relation dir refactor work.

If we do not create images as in this patch, we would get two set of image layers:

0000...METADATA_KEYS
0000...REL_KEYS

They overlap at the same LSN and would cause data loss for relation keys. This doesn't happen in prod because initial image layer generation is never called, but better to be fixed to avoid future issues with the reldir refactors.

Summary of changes

• Consolidate create_image_layers call into a single one.

[github-actions]

7304 tests run: 6934 passed, 0 failed, 370 skipped (full report)

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Code coverage* (full report)

• functions: 32.7% (8056 of 24666 functions)
• lines: 47.8% (67022 of 140341 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
877671e at 2025-01-13T20:26:43.255Z :recycle:}

[problame]

LGTM.

Should we should just remove the initdb_lsn => image layer optimization if it's dead at runtime?

[skyzh]

Should we should just remove the initdb_lsn => image layer optimization if it's dead at runtime?

I think it still makes some sense so let's keep it until the code is unmaintainable.