test: no missed wakeups, cancellation and timeout flow to downloads

[koivunej]

I suspected a wakeup could be lost with remote_storage::support::DownloadStream if the cancellation and inner stream wakeups happen simultaneously. The next poll would only return the cancellation error without setting the wakeup. There is no lost wakeup because the single future for getting the cancellation error is consumed when the value is ready, and a new future is created for the next value. The new future is always polled. Similarly, if only the Stream::poll_next is being used after a Some(_) value has been yielded, it makes no sense to have an expectation of a wakeup for the (N+1)th stream value already set because when a value is wanted, Stream::poll_next will be called.

A test is added to show that the above is true.

Additionally, there was a question of these cancellations and timeouts flowing to attached or secondary tenant downloads. A test is added to show that this, in fact, happens.

Lastly, a warning message is logged when a download stream is polled after a timeout or cancellation error (currently unexpected) so we can rule it out while troubleshooting.

[github-actions]

3186 tests run: 3047 passed, 0 failed, 139 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_pageserver_restarts_under_worload: release

Code coverage* (full report)

• functions: 31.5% (6573 of 20866 functions)
• lines: 48.4% (50863 of 104992 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
700c12d at 2024-06-04T09:39:53.155Z :recycle:}