Improve slow operations observability in safekeepers

[petuhovskiy]

After #8022 was deployed to staging, I noticed many cases of timeouts. After inspecting the logs, I realized that some operations are taking ~20 seconds and they're doing while holding shared state lock. Usually it happens right after redeploy, because compute reconnections put high load on disks. This commit tries to improve observability around slow operations.

Non-observability changes:

• TimelineState::finish_change now skips update if nothing has changed
• wal_residence_guard() timeout is set to 30s

[github-actions]

2940 tests run: 2823 passed, 0 failed, 117 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_lr_with_slow_safekeeper: release

Code coverage* (full report)

• functions: 32.6% (6896 of 21128 functions)
• lines: 50.0% (53957 of 107939 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
53c0746 at 2024-06-27T16:45:54.462Z :recycle:}