pageserver: cancellation for remote ops in tenant deletion on shutdown

[jcsp]

Problem

Tenant deletion had a couple of TODOs where we weren't using proper cancellation tokens that would have aborted the deletions during process shutdown.

Summary of changes

• Refactor enough that deletion/shutdown code has access to the TenantManager's cancellation toke
• Use that cancellation token in tenant deletion instead of dummy tokens.

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]

2712 tests run: 2586 passed, 1 failed, 125 skipped (full report)

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Failures on Postgres 14

• test_sharding_split_failures[failure10]: debug

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_sharding_split_failures[debug-pg14-failure10]"

Flaky tests (2)

Postgres 14

• test_pageserver_recovery: debug
• test_empty_tenant_size: debug

Code coverage* (full report)

• functions: 28.3% (7036 of 24824 functions)
• lines: 47.0% (43467 of 92448 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
406317b at 2024-03-15T18:20:59.131Z :recycle:}