pageserver: finish vectored get early

[VladLazar]

Problem

If the previous step of the vectored left no further keyspace to investigate
(i.e. keyspace remains empty after removing keys completed in the previous step),
then we'd still grab the layers lock, potentially add an in-mem layer to the fringe and
at some further point read its index without reading any values from it.

Summary of changes

If there's nothing left in the current keyspace, then skip the search and just select
the next item from the fringe as usual.

When running test_pg_regress[release-pg16] with the vectored read path for singular
gets this improved perf drastically:

Legend:
blue - without this patch
orange - with this patch

Correctness

Since no keys remained from the previous range (i.e. we are on a leaf node) there's nothing
that search can find in deeper nodes.

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]

2766 tests run: 2645 passed, 0 failed, 121 skipped (full report)

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Code coverage* (full report)

• functions: 28.1% (6479 of 23038 functions)
• lines: 47.0% (46025 of 97941 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e57853d at 2024-04-24T13:52:34.009Z :recycle:}

[arpad-m]

Good catch!