pageserver: revert concurrent secondary downloads

[jcsp]

Problem

Ongoing hunt for secondary location shutdown hang issues.

Summary of changes

• Revert the functional changes from pageserver: use adaptive concurrency in secondary layer downloads #7675
• Tweak a log in secondary downloads to make it more apparent when we drop out on cancellation
• Modify DownloadStream's behavior to always return an Err after it has been cancelled. This should not impact anything, but it makes the behavior simpler to reason about (e.g. even if the poll function somehow got called again, it could never end up in an un-cancellable state)

Related #https://github.com/neondatabase/cloud/issues/13576

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]

3114 tests run: 2987 passed, 0 failed, 127 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_statvfs_pressure_usage: release

Code coverage* (full report)

• functions: 31.4% (6443 of 20532 functions)
• lines: 48.3% (49787 of 103138 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
263f2a8 at 2024-05-24T08:49:01.203Z :recycle:}

[koivunej]

Let's just revert, no additional changes.

[jcsp]

Let's just revert, no additional changes.

I've trimmed this PR to just the revert & secondary mode log change.