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btcclient+btcjson: add type alias for BTC/kvB

Added type alias BTC/kvB to explicitly indicate that
it represents the fee in BTC for a transaction size of 1 kB.
Because bitcoind uses its own fee rate type
(BTC/kvB instead of sat/kWU we use in lnd),
define the type in btcjson package,
as it's the only place where we actually use BTC/kvB.