pageserver: return 429 on timeline creation in progress

[jcsp]

Problem

Currently, we return 409 (Conflict) in two cases:

• Temporary: Timeline creation cannot proceed because another timeline with the same ID is being created
• Permanent: Timeline creation cannot proceed because another timeline exists with different parameters but the same ID.

Callers which time out a request and retry should be able to distinguish these cases.

Closes: #7208

Summary of changes

• Expose AlreadyCreating errors as 429 instead of 409

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]

2742 tests run: 2604 passed, 0 failed, 138 skipped (full report)

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Flaky tests (3)

Postgres 15

• test_multi_attach: debug
• test_vm_bit_clear_on_heap_lock: debug

Postgres 14

• test_metrics_while_ignoring_broken_tenant_and_reloading: debug

Code coverage* (full report)

• functions: 28.1% (6287 of 22345 functions)
• lines: 47.0% (44227 of 94111 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
f51adfc at 2024-03-25T19:00:04.851Z :recycle:}

[problame]

I trust you checked that the control plane client does the right thing. Not obvious to me that they are aware, given the Slack thread linked in the issue.