feat(pageserver): rewrite streaming vectored read planner

[skyzh]

Problem

Rewrite streaming vectored read planner to be a separate struct. The API is designed to produce batches around max_read_size instead of exactly less than that so that handle_XX returns one batch a time.

Summary of changes

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]

3061 tests run: 2946 passed, 0 failed, 115 skipped (full report)

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Flaky tests (3)

Postgres 16

• test_tenant_creation_fails: debug

Postgres 15

• test_pg_regress[4]: debug

Postgres 14

• test_tenant_creation_fails: debug

Code coverage* (full report)

• functions: 32.8% (6974 of 21287 functions)
• lines: 50.2% (54913 of 109430 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
55da0a3 at 2024-07-11T18:36:15.824Z :recycle:}

[VladLazar]

Looks good. I'd like to get rid of that VectoredReadBuilder constructor if possible.

[VladLazar]

pg_regress failure is strange, but I don't see anything in this pr could have caused it.
Might be good to note down the link to the failure somewhere