pageserver: fix early bail out in vectored get

[VladLazar]

Problem

When vectored get encountered a portion of the key range that could
not be mapped to any layer in the current timeline it would incorrectly
bail out of the current timeline. This is incorrect since we may have
had layers queued for a visit in the fringe.

This was observed in staging on Peter's benchmark tenants.

Summary of changes

• Add a repro unit test
• Remove the early bail out path
• Simplify range search return value

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]

2490 tests run: 2370 passed, 0 failed, 120 skipped (full report)

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Code coverage* (full report)

• functions: 28.9% (7002 of 24268 functions)
• lines: 47.6% (43219 of 90865 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
ac3c778 at 2024-03-07T13:32:26.634Z :recycle:}

[koivunej]

Looking good!