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

• [x] 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[bot]]

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](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9051/10947226909/index.html#suites/f08716c3eb261a35d11a79ed9535ded6/18240e6e58c497ff/retries) - `test_ondemand_wal_download_in_replication_slot_funcs`: [release-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9051/10947226909/index.html#suites/180444c850d4a41d41eb0a410dc16d84/a15d7f525bcecdd1/retries) - `test_scrubber_physical_gc[4]`: [debug-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9051/10947226909/index.html#suites/616e84f65c91fe4bc748db7447d35268/136c7c7b403cd6b7/retries) #### Postgres 16 - `test_slots_and_branching`: [release-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9051/10947226909/index.html#suites/180444c850d4a41d41eb0a410dc16d84/9713a8e993a58242/retries) - `test_obsolete_slot_drop[cross-validation]`: [release-arm64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9051/10947226909/index.html#suites/180444c850d4a41d41eb0a410dc16d84/5aa6775906f58a8e/retries) #### Postgres 15 - `test_layer_download_cancelled_by_config_location`: [release-arm64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9051/10947226909/index.html#suites/b97efae3a617afb71cb8142f5afa5224/6a69b19d74d2fd63/retries) - `test_scrubber_physical_gc[4]`: [release-arm64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9051/10947226909/index.html#suites/616e84f65c91fe4bc748db7447d35268/bbe982ae19534437/retries) - `test_subscriber_restart`: [release-x86-64](https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9051/10947226909/index.html#suites/8be0c222d5601535470e7e5978bbfb03/9e07b2f8d0678a6f/retries)

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
f8d8f5b90fa487a9e82c42da223f012f5d4fece7 at 2024-09-19T20:39:10.603Z :recycle:}

[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.