compute hook: use shared client with explicit timeout

[jcsp]

Problem

We are seeing some mysterious long waits when sending requests.

Summary of changes

• To eliminate risk that we are incurring some unreasonable overheads from setup, e.g. DNS, use a single Client (internally a pool) instead of repeatedly constructing a fresh one.
• To make it clearer where a timeout is occurring, apply a 10 second timeout to requests as we send them.

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]

2754 tests run: 2636 passed, 0 failed, 118 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_ts_of_lsn_api: debug

Code coverage* (full report)

• functions: 28.1% (6429 of 22866 functions)
• lines: 46.8% (45140 of 96404 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
69e1c64 at 2024-04-11T14:27:41.242Z :recycle:}