pageserver: tweak oversized key read path warning

[VladLazar]

Problem

Oversized vectored read [...] logs are spewing in prod because we have a few keys that
are unexpectedly large:

• reldir/relblock - these are unbounded, so it's known technical debt
• slru block - they can be a bit bigger than 128KiB due to storage format overhead

Summary of changes

• Bump threshold to 130KiB
• Don't warn on oversized reldir and dbdir keys

Closes #8967

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]

5084 tests run: 4897 passed, 1 failed, 186 skipped (full report)

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Failures on Postgres 17

• test_storage_controller_heartbeats[failure3]: debug-x86-64

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_storage_controller_heartbeats[debug-pg17-failure3]"

Flaky tests (2)

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64

Postgres 15

• test_subscriber_restart: release-x86-64

Code coverage* (full report)

• functions: 31.4% (7491 of 23884 functions)
• lines: 49.5% (60135 of 121370 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
1464ba1 at 2024-10-03T14:50:40.412Z :recycle:}

[koivunej]

Smaller notes... I guess we are always bound, but prefer the additional comments and display impl.