proxy: cull http connections

[conradludgate]

Problem

Some HTTP client connections can stay open for quite a long time.

Summary of changes

When there are too many HTTP client connections, pick a random connection and gracefully cancel it.

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]

2970 tests run: 2843 passed, 0 failed, 127 skipped (full report)

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Flaky tests (4)

Postgres 15

• test_partial_evict_tenant[relative_equal]: release
• test_gc_aggressive: debug
• test_lock_time_tracing: release

Postgres 14

• test_empty_branch_remote_storage_upload: debug

Code coverage* (full report)

• functions: 31.2% (6246 of 19995 functions)
• lines: 46.7% (46770 of 100184 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
50a45e9 at 2024-05-07T16:52:56.158Z :recycle:}