fix(pageserver): ensure sparse keyspace is ordered

[skyzh]

Problem

fix #8277

Summary of changes

make sure the sparse keyspace has ordered keyspace parts

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]

3042 tests run: 2927 passed, 0 failed, 115 skipped (full report)

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Code coverage* (full report)

• functions: 32.6% (6935 of 21276 functions)
• lines: 50.0% (54509 of 108989 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6b5e118 at 2024-07-05T16:40:15.656Z :recycle:}