wallet: add 'spendable_balance' to getwalletinfo RPC

[ca98am79]

This enables a user to view their spendable balance using the getwalletinfo RPC

[coveralls]

Pull Request Test Coverage Report for Build 232854351

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.8%) to 59.161%

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Files with Coverage Reduction	New Missed Lines	%
lib/protocol/consensus.js	1	82.79%

Totals
Change from base Build 212167982:	0.8%
Covered Lines:	19359
Relevant Lines:	30452

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💛 - Coveralls

[nodech]

I am wondering if we should modify this and/or add getbalancedetails(or info) rpc methods to include balance details which can be used by users however they want. It could be advanced version of getbalance.

[pinheadmz]

I am wondering if we should modify this and/or add getbalancedetails(or info) rpc methods to include balance details which can be used by users however they want. It could be advanced version of getbalance.

Bitcoin core has extra details in getwalletinfo and also has separate rpc getwalletbalances which also calculates the immature newly mined coins (see #662)