rpcclient: ensure http dial respects timeout

[wydengyre]

Change Description

Ensure that the dial phase of rpcclient's HTTP posts respects the http.Client's Timeout. It was instead falling back to the OS settings.

Steps to Test

This is a bit challenging to test, as it requires simulating a very laggy dial. Not sure the juice is worth the squeeze.

Pull Request Checklist

Testing

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• Tests covering the positive and negative (error paths) are included.
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[coveralls]

Pull Request Test Coverage Report for Build 18913980728

Details

• 1 of 4 (25.0%) changed or added relevant lines in 1 file are covered.
• 16 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.09%) to 54.954%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcclient/infrastructure.go	1	4	25.0%

Files with Coverage Reduction	New Missed Lines	%
rpcclient/infrastructure.go	16	47.8%

Totals
Change from base Build 18858421588:	0.09%
Covered Lines:	31199
Relevant Lines:	56773

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💛 - Coveralls

[wydengyre]

Note, it looks like the underlying issue here was introduced in https://github.com/btcsuite/btcd/pull/2168/files

[Roasbeef]

LGTM 🐩