pageserver: drop the layer map lock after planning reads

[VladLazar]

Problem

The vectored read path holds the layer map lock while visiting a timeline.

Summary of changes

• Rework the fringe order to hold Layer on Arc<InMemoryLayer> handles instead of descriptions that are resolved by the layer map at the time of read. Note that previously get_values_reconstruct_data was implemented for the layer description which already knew the lsn range for the read. Now it is implemented on the new ReadableLayer handle and needs to get the lsn range as an argument.
• Drop the layer map lock after updating the fringe.

Related #6833

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]

2730 tests run: 2590 passed, 0 failed, 140 skipped (full report)

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Code coverage* (full report)

• functions: 28.2% (6307 of 22365 functions)
• lines: 47.0% (44298 of 94281 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
b1feb1f at 2024-03-27T16:28:03.162Z :recycle:}

[koivunej]

Accidentially hit the pending review comment button, flushing.

[problame]

Seems like we can remove the InMemoryLayerHandle?
It's dead code, but not identified because it's pub.

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I haven't reviewed the Ordering / descriptor stuff, as Joonas has already commented on it.
I think it would be nice to minimize this diff by

1. putting the new types in the same location in the file as the old types and
2. using the same names

We can improve the names in a later PR.

Then again, this is not a super strongly held request.

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The PR description only mentions "rework the fringe a bit", but, doesn't explain why the signature of get_values_reconstruct_data had to change and now includes an LSN range. I think a bit more description is justified.

Specifically, it would be nice to explain what exactly has been reworked.

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The core change "hold struct Layer or Arc" without the layer map lock is sound.

[VladLazar]

Seems like we can remove the InMemoryLayerHandle? It's dead code, but not identified because it's pub.

Good catch. Done.

I haven't reviewed the Ordering / descriptor stuff, as Joonas has already commented on it. I think it would be nice to minimize this diff by

1. putting the new types in the same location in the file as the old types and

Done.

2. using the same names

The old names don't really fit anymore.

We can improve the names in a later PR.

Then again, this is not a super strongly held request.

The PR description only mentions "rework the fringe a bit", but, doesn't explain why the signature of get_values_reconstruct_data had to change and now includes an LSN range. I think a bit more description is justified.

Specifically, it would be nice to explain what exactly has been reworked.

Updated the PR description.