pageserver: do fewer heatmap uploads for tiny tenants

[jcsp]

Problem

Currently we do a large number of heatmap uploads for tiny tenants. "tiny" in this context is defined as being less than a single layer in size. These uploads are triggered by atime changes rather than changes in the set of layers.

Uploading heatmaps for atime changes on small tenants isn't useful, because even without bumping these atimes, disk usage eviction still avoids evicting the largest resident layer of a tenant, which in practice keeps tiny/empty tenants mostly resident irrespective of atimes.

Summary of changes

• For tenants smaller than one checkpoint interval, only upload heatmap if the set of layers has changed, not if only the atimes have changed.
• Include the heatmap period in the uploaded heatmap, as a precursor to implementing pageserver: optimize heatmap downloads by passing period/next_upload in the heatmap #6200 (auto-adjusting download intervals to match upload intervals)

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]

3060 tests run: 2927 passed, 0 failed, 133 skipped (full report)

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Code coverage* (full report)

• functions: 31.4% (6329 of 20167 functions)
• lines: 47.3% (47748 of 100999 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
4939e47 at 2024-05-13T13:03:03.545Z :recycle:}