rpcclient: implement gettxoutsetinfo command

[onyb]

#RunTheNumbers seems to be a thing on Twitter these days. This PR adds the gettxoutsetinfo command to the rpcclient package, which is useful for performing inflation checks in Bitcoin.

[coveralls]

Pull Request Test Coverage Report for Build 276658531

Warning: This coverage report may be inaccurate.

We've detected an issue with your CI configuration that might affect the accuracy of this pull request's coverage report.
To ensure accuracy in future PRs, please see these guidelines.
A quick fix for this PR: rebase it; your next report should be accurate.

• 25 of 49 (51.02%) changed or added relevant lines in 2 files are covered.
• 38 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.03%) to 53.554%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
btcjson/chainsvrresults.go	25	31	80.65%
rpcclient/chain.go	0	18	0.0%

Files with Coverage Reduction	New Missed Lines	%
btcec/signature.go	3	92.48%
peer/peer.go	9	75.47%
btcjson/chainsvrresults.go	26	57.05%

Totals
Change from base Build 272743387:	-0.03%
Covered Lines:	20756
Relevant Lines:	38757

---

💛 - Coveralls

[jcvernaleo]

OK