safekeeper: move is_cancelled to write_shared_state.

[arssher]

Each time we access shared state we either want to error out if it is cancelled or ignore that; make that explicit.

[github-actions]

3024 tests run: 2891 passed, 0 failed, 133 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_secondary_background_downloads: debug

Code coverage* (full report)

• functions: 31.4% (6319 of 20136 functions)
• lines: 47.3% (47617 of 100708 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
5e650c8 at 2024-05-10T04:50:21.670Z :recycle:}

[petuhovskiy]

I don't understand why is it better.

[arssher]

It enforces checking for cancelled timeline: each time you lock it, you must think whether to allow cancelled or not. Also, with more modes which disallow access (paused in next pr, probably evicted timelines) it also becomes less verbose.