refactor(pageserver main): signal handling

[problame]

This refactoring makes it easier to experimentally replace BACKGROUND_RUNTIME with a single-threaded runtime. Found this useful during benchmarking.

[github-actions]

2376 tests run: 2283 passed, 0 failed, 93 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_gc_cutoff: debug, release

Postgres 15

• test_crafted_wal_end[last_wal_record_crossing_segment]: release

Postgres 14

• test_gc_cutoff: release

Code coverage (full report)

• functions: 54.6% (11223 of 20570 functions)
• lines: 81.6% (62997 of 77226 lines)

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_{The comment gets automatically updated with the latest test results
92df353 at 2024-01-31T23:40:13.475Z :recycle:}