pageserver: add wait_until_flushed parameter for timeline checkpoint

[erikgrinaker]

Problem

I'm writing an ingest benchmark in #9812. To time S3 uploads, I need to schedule a flush of the Pageserver's in-memory layer, but don't actually want to wait around for it to complete (which will take a minute).

Summary of changes

Add a parameter wait_until_flush (default true) for timeline/checkpoint to control whether to wait for the flush to complete.

[github-actions]

7040 tests run: 6731 passed, 0 failed, 309 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_pageserver_gc_compaction_smoke: release-x86-64

Postgres 15

• test_pageserver_gc_compaction_smoke: release-x86-64
• test_pull_timeline[True]: release-x86-64

Code coverage* (full report)

• functions: 31.4% (8324 of 26503 functions)
• lines: 47.8% (65442 of 137029 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6d735cf at 2024-12-06T10:07:23.157Z :recycle:}