pageserver: suppress noisy "layer became visible" logs

[jcsp]

Problem

When layer visibility was added, an info log was included for the situation where actual access to a layer disagrees with the visibility calculation. This situation is safe, but I was interested in seeing when it happens.

The log is pretty high volume, so this PR refines it to fire less often.

Summary of changes

• For cases where accessing non-visible layers is normal, don't log at all.
• Extend a unit test to increase confidence that the updates to visibility on access are working as expected
• During compaction, only call the visibility calculation routine if some image layers were created: previously, frequent calls resulted in the visibility of layers getting reset every time we passed through create_image_layers.

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]

4994 tests run: 4830 passed, 0 failed, 164 skipped (full report)

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Flaky tests (6)

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, release-arm64

Postgres 16

• test_subscriber_restart: release-x86-64
• test_timeline_delete_works_for_remote_smoke: release-arm64

Code coverage* (full report)

• functions: 31.9% (7424 of 23300 functions)
• lines: 49.8% (59749 of 119917 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
53437f7 at 2024-09-20T16:17:15.785Z :recycle:}