pageserver: extra debug for test_secondary_downloads failures

[jcsp]

• Enable debug logs for this test
• Add some debug logging detail in downloader.rs
• Add an info-level message in scheduler.rs that makes it obvious if a command is waiting for an existing task rather than spawning a new one.

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]

2706 tests run: 2575 passed, 0 failed, 131 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_vm_bit_clear_on_heap_lock: debug

Postgres 15

• test_timeline_size_quota_on_startup: release

Code coverage* (full report)

• functions: 28.3% (7131 of 25202 functions)
• lines: 46.9% (43712 of 93300 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c1691e6 at 2024-03-20T11:26:06.380Z :recycle:}