Add listunspent RPC call

Returns a brief json summary of each utxo found by calling
ListUnspentWitness in the wallet. The two arguments are the
minimum and maximum number of conrfirmations (0=include
unconfirmed)

lnwallet/btcwallet/btcwallet.go

@@ -349,6 +349,7 @@ func (b *BtcWallet) ListUnspentWitness(minConfs, maxConfs int32) (
 					Hash:  *txid,
 					Index: output.Vout,
 				},
+				Confirmations: output.Confirmations,
 			}
 			witnessOutputs = append(witnessOutputs, utxo)
 		}

lnwallet/interface.go

@@ -52,6 +52,7 @@ var (
 type Utxo struct {
 	AddressType   AddressType
 	Value         btcutil.Amount
+	Confirmations int64
 	PkScript      []byte
 	RedeemScript  []byte
 	WitnessScript []byte