rpcserver: add GetBlockStats

[guilhermep]

Adds GetBlockStats implementation to rpcserver.

This is my first PR. Any feedback is appreciated.

Fixes #1630 (partially).

[jcvernaleo]

Thanks for the PR!

Could you fix it so the tests pass before we review? Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 399629124

• 0 of 345 (0.0%) changed or added relevant lines in 1 file are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.5%) to 52.934%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcserver.go	0	345	0.0%
<!--	Total:	0	345

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	2	75.47%
btcec/signature.go	3	92.48%
connmgr/connmanager.go	3	86.07%
<!--	Total:	8

Totals
Change from base Build 381748156:	-0.5%
Covered Lines:	20828
Relevant Lines:	39347

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💛 - Coveralls

[jcvernaleo]

Thanks for the lint fixing. Looks like it has conflicts though. Could you rebase and fix those please?