refactor(pageserver): check layer map valid in one place

[skyzh]

Problem

We have 3 places where we implement layer map checks.

Summary of changes

Now we have a single check function being called in all places.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5020 tests run: 4856 passed, 0 failed, 164 skipped (full report)

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Flaky tests (8)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64
• test_scrubber_physical_gc[4]: debug-x86-64

Postgres 16

• test_slots_and_branching: release-x86-64
• test_obsolete_slot_drop[cross-validation]: release-arm64

Postgres 15

• test_layer_download_cancelled_by_config_location: release-arm64
• test_scrubber_physical_gc[4]: release-arm64
• test_subscriber_restart: release-x86-64

Code coverage* (full report)

• functions: 31.9% (7423 of 23300 functions)
• lines: 49.8% (59764 of 119904 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
f8d8f5b at 2024-09-19T20:39:10.603Z :recycle:}

[koivunej]

I was a bit worried you were thinking of doing this with the prod code, approved with the added cfg(test).

[skyzh]

I was a bit worried you were thinking of doing this with the prod code, approved with the added cfg(test).

The code is also used in storage-scrubber so it's not test only...

[koivunej]

test_layer_download_timeouted: release-x86-64

This looks surprising, I'll peek. You can just re-run failed or push it's unrelated, but I haven't seen this failure mode.