pageserver: reduce runtime of init_tenant_mgr

[jcsp]

Problem

init_tenant_mgr blocks the rest of pageserver startup, including starting the admin API.

This was noticeable in #7475 , where the init_tenant_mgr runtime could be long enough to trip the controller's 30 second heartbeat timeout.

Summary of changes

• When detaching tenants during startup, spawn the background deletes as background tasks instead of doing them inline
• Write all configs before spawning any tenants, so that the config writes aren't fighting tenants for system resources
• Write configs with some concurrency (16) rather than writing them all sequentially.

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]

2850 tests run: 2729 passed, 0 failed, 121 skipped (full report)

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Code coverage* (full report)

• functions: 28.2% (6573 of 23345 functions)
• lines: 46.9% (46723 of 99667 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
047d9a2 at 2024-04-30T09:33:24.463Z :recycle:}