sweep: catch third party spent in fee bumper for neutrino

This commit adds a new check for neutrino backend - when the inputs in
the sweeping tx are spent by a third party, we will send back a
`TxFailed` event to free the rest of the inputs for re-grouping.

sweep/interface.go

@@ -51,4 +51,9 @@ type Wallet interface {
 	// its transaction hash.
 	GetTransactionDetails(txHash *chainhash.Hash) (
 		*lnwallet.TransactionDetail, error)
+
+	// BackEnd returns a name for the wallet's backing chain service,
+	// which could be e.g. btcd, bitcoind, neutrino, or another consensus
+	// service.
+	BackEnd() string
 }