pageserver: remove AdjacentVectoredReadBuilder and bump minmimum io_buffer_alignment to 512

[yliang412]

Part of #8130

Problem

After deploying https://github.com/neondatabase/infra/pull/1927, we shipped io_buffer_alignment=512 to all prod region. The AdjacentVectoredReadBuilder code path is no longer taken and we are running pageserver unit tests 6 times in the CI. Removing it would reduce the test duration by 30-60s.

Summary of changes

• Remove AdjacentVectoredReadBuilder code.
• Bump the minimum io_buffer_alignment requirement to at least 512 bytes.
• Use default io_buffer_alignment for Rust unit tests.

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]

5013 tests run: 4855 passed, 0 failed, 158 skipped (full report)

---

Flaky tests (7)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_pg_regress[None]: debug-x86-64
• test_subscriber_restart: release-x86-64, debug-x86-64

Postgres 16

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

Postgres 15

• test_ondemand_wal_download_in_replication_slot_funcs: release-arm64

Code coverage* (full report)

• functions: 32.0% (7487 of 23412 functions)
• lines: 50.0% (60486 of 120990 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
677f9a4 at 2024-09-27T15:37:13.458Z :recycle:}