refactor(pageserver) remove task_mgr for most global tasks

[problame]

Motivation & Context

We want to move away from task_mgr towards explicit tracking of child tasks.

This PR is extracted from #8339 where I refactor PageRequestHandler to not depend on task_mgr anymore.

Changes

This PR refactors all global tasks but PageRequestHandler to use some combination of JoinHandle/JoinSet + CancellationToken.

The task_mgr::spawn(.., shutdown_process_on_error) functionality is preserved through the new exit_on_panic_or_error wrapper.
Some global tasks were not using it before, but as of this PR, they are.
The rationale is that all global tasks are relevant for correct operation of the overall Neon system in one way or another.

Future Work

After #8339, we can make task_mgr::spawn require a TenantId instead of an Option<TenantId> which concludes this step of cleanup work and will help discourage future usage of task_mgr for global tasks.

Notes for Reviewers

The individual commits are mostly orthogonal but it's too painful to stack them in a series of PRs.
I would recommend reviewing commit-by-commit.

[github-actions]

3144 tests run: 3023 passed, 0 failed, 121 skipped (full report)

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Code coverage* (full report)

• functions: 32.6% (7003 of 21475 functions)
• lines: 50.0% (55249 of 110491 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
fbf320b at 2024-07-22T11:35:20.953Z :recycle:}