pageserver: jitter secondary periods

[jcsp]

Problem

After some time the load from heatmap uploads gets rather spiky. They're unintentionally synchronising.

Chart (does this make a boing sound in anyone else's head?):

Summary of changes

• Add a helper period_jitter and apply a 5% jitter from downloader and heatmap_uploader when updating the next runtime at the end of an interation.
• Refactor existing places that we pick a startup interval into period_warmup, so that the intent is obvious.

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]

2868 tests run: 2747 passed, 0 failed, 121 skipped (full report)

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Flaky tests (2)

Postgres 15

• test_vm_bit_clear_on_heap_lock: debug

Postgres 14

• test_partial_evict_tenant[relative_spare]: release

Code coverage* (full report)

• functions: 28.0% (6618 of 23601 functions)
• lines: 46.7% (47046 of 100674 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
093dcb3 at 2024-05-03T12:46:57.409Z :recycle:}

[koivunej]

Difficult to say if these changes will actually get rid of the problem because of me not fully understanding the scheduling framework framework, but cannot see how these could hurt.