rpcclient: make shutdown interrupt in-flight POSTs

[wydengyre]

Change Description

Modify the rpcclient http POST call to ensure that a shutdown immediately interrupts in-flight requests, which otherwise would have to wait until timeout.

This PR includes the change in #2450 as it is necessary to ensure interruption during Dial.

Steps to Test

The test suite has an added test to ensure this works.

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.

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[coveralls]

Pull Request Test Coverage Report for Build 19126705309

Details

• 21 of 28 (75.0%) changed or added relevant lines in 1 file are covered.
• 93 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.006%) to 54.967%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcclient/infrastructure.go	21	28	75.0%

Files with Coverage Reduction	New Missed Lines	%
mempool/mempool.go	1	66.56%
btcutil/gcs/gcs.go	2	81.55%
connmgr/connmanager.go	2	83.9%
rpcclient/infrastructure.go	88	48.35%

Totals
Change from base Build 19121431262:	0.006%
Covered Lines:	31216
Relevant Lines:	56790

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💛 - Coveralls