Add getchaintips rpc call

[coveralls]

Pull Request Test Coverage Report for Build 3691652504

• 81 of 117 (69.23%) changed or added relevant lines in 4 files are covered.
• 12 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.03%) to 53.426%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcclient/chain.go	0	18	0.0%
rpcserver.go	0	18	0.0%
<!--	Total:	81	117	69.23%	-->

Files with Coverage Reduction	New Missed Lines	%
addrmgr/network.go	2	73.58%
btcec/signature.go	3	92.76%
peer/peer.go	3	74.9%
database/ffldb/blockio.go	4	93.68%
<!--	Total:	12		-->

Totals
Change from base Build 3691535285:	0.03%
Covered Lines:	27362
Relevant Lines:	51215

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💛 - Coveralls