proxy: fix password hash cancellation

[conradludgate]

In #8863 I replaced the threadpool with tokio tasks, but there was a behaviour I missed regarding cancellation. Adding the JoinHandle wrapper that triggers abort on drop should fix this.

Another change, any panics that occur in password hashing will be propagated through the resume_unwind functionality.

[github-actions]

4061 tests run: 3955 passed, 0 failed, 106 skipped (full report)

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Flaky tests (11)

Postgres 16

• test_backward_compatibility: release-x86-64, debug-x86-64, release-arm64
• test_subscriber_restart: release-x86-64, debug-x86-64

Postgres 15

• test_backward_compatibility: release-x86-64, release-arm64
• test_location_conf_churn[2]: release-x86-64
• test_pull_timeline_partial_segment_integrity: release-x86-64

Postgres 14

• test_backward_compatibility: release-x86-64, release-arm64

Code coverage* (full report)

• functions: 32.5% (7413 of 22809 functions)
• lines: 50.6% (60048 of 118588 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
63d9e7c at 2024-08-29T19:21:30.788Z :recycle:}