feat(pageserver): add iterator API for btree reader

[skyzh]

Problem

The new image iterator and delta iterator uses an iterator-based API. #8006 / part of #8002

This requires the underlying thing (the btree) to have an iterator API, and the iterator should have a type name so that it can be stored somewhere.

pub struct DeltaLayerIterator {
  index_iterator: BTreeIterator
}

versus

pub struct DeltaLayerIterator {
  index_iterator: impl Stream<....>
}

(this requires nightly flag and still buggy in the Rust compiler)

There are multiple ways to achieve this:

1. Either write a BTreeIterator from scratch that provides async next. This is the most efficient way to do that.
2. Or wrap the current get_stream API, which is the current approach in the pull request.

In the future, we should do (1), and the get_stream API should be refactored to use the iterator API. With (2), we have to wrap the get_stream API with Pin<Box<dyn Stream>>, where we have the overhead of dynamic dispatch. However, (2) needs a rewrite of the visit function, which would take some time to write and review. I'd like to define this iterator API first and work on a real iterator API later.

Summary of changes

Add DiskBtreeIterator and related 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]

3222 tests run: 3105 passed, 0 failed, 117 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_wal_restore_initdb: release

Code coverage* (full report)

• functions: 32.4% (6842 of 21103 functions)
• lines: 50.0% (53401 of 106835 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
24462d1 at 2024-06-18T19:36:57.961Z :recycle:}

[VladLazar]

Looks good to me but clippy is complaining