pageserver: use histogram for background job semaphore waits

[erikgrinaker]

Problem

We don't have visibility into how long an individual background job is waiting for a semaphore permit.

Summary of changes

• Make pageserver_background_loop_semaphore_wait_seconds a histogram rather than a sum.
• Add a paced warning when a task takes more than 10 minutes to get a permit (for now).
• Drive-by cleanup of some EnumMap usage.

[github-actions]

7425 tests run: 7071 passed, 0 failed, 354 skipped (full report)

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Code coverage* (full report)

• functions: 33.3% (8589 of 25823 functions)
• lines: 49.1% (72294 of 147250 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
f6f185b at 2025-02-06T17:26:11.544Z :recycle:}