pageserver: simple fix for vectored read image layer skip

[VladLazar]

Problem

Different keyspaces may require different floor LSNs in vectored
delta layer visits. This patch adds support for such cases.

Summary of changes

Different keyspaces wishing to read the same layer might
require different stop lsns (or lsn floor). The start LSN
of the read (or the lsn ceil) will always be the same.

With this observation, we fix skipping of image layers by
indexing the fringe by layer id plus lsn floor.

This is very simple, but means that we can visit delta layers twice
in certain cases. Still, I think it's very unlikely for any extra merging
to have taken place in this case, so perhaps it makes sense to go
with the simpler patch.

Fixes #9012
Alternative to #9025

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]

5072 tests run: 4908 passed, 0 failed, 164 skipped (full report)

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Flaky tests (11)

Postgres 17

• test_hot_standby_feedback: debug-x86-64
• test_scrubber_physical_gc[4]: release-arm64, debug-x86-64

Postgres 16

• test_neon_cli_basics: release-arm64
• test_scrubber_physical_gc[4]: release-arm64
• test_subscriber_restart: release-arm64

Postgres 15

• test_ondemand_wal_download_in_replication_slot_funcs: release-arm64, release-x86-64
• test_subscriber_restart: release-x86-64

Postgres 14

• test_lfc_resize: release-arm64
• test_neon_cli_basics: release-arm64

Code coverage* (full report)

• functions: 31.8% (7422 of 23309 functions)
• lines: 49.8% (59697 of 119797 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
60451ca at 2024-09-19T12:58:58.061Z :recycle:}