multi: switch to transaction weight to calculate transaction fees

Due to a recent change within the codebase to return estimated fee rates
in sat/kw, this commit ensures that we use this fee rate properly by
calculing a transaction's fees using its weight. This includes all of
the different transactions that are created within lnd (funding, sweeps,
etc.). On-chain transactions still rely on a sat/vbyte fee rate since it's
required by btcwallet.

utxonursery.go

@@ -994,15 +994,15 @@ func (u *utxoNursery) createSweepTx(kgtnOutputs []kidOutput,
 	utxnLog.Infof("Creating sweep transaction for %v CSV inputs, %v CLTV "+
 		"inputs", len(csvOutputs), len(cltvOutputs))
 
-	txVSize := int64(weightEstimate.VSize())
-	return u.populateSweepTx(txVSize, classHeight, csvOutputs, cltvOutputs)
+	txWeight := int64(weightEstimate.Weight())
+	return u.populateSweepTx(txWeight, classHeight, csvOutputs, cltvOutputs)
 }
 
 // populateSweepTx populate the final sweeping transaction with all witnesses
 // in place for all inputs using the provided txn fee. The created transaction
 // has a single output sending all the funds back to the source wallet, after
 // accounting for the fee estimate.
-func (u *utxoNursery) populateSweepTx(txVSize int64, classHeight uint32,
+func (u *utxoNursery) populateSweepTx(txWeight int64, classHeight uint32,
 	csvInputs []CsvSpendableOutput,
 	cltvInputs []SpendableOutput) (*wire.MsgTx, error) {
 
@@ -1022,11 +1022,11 @@ func (u *utxoNursery) populateSweepTx(txVSize int64, classHeight uint32,
 	}
 
 	// Using the txn weight estimate, compute the required txn fee.
-	feePerVSize, err := u.cfg.Estimator.EstimateFeePerVSize(6)
+	feePerKw, err := u.cfg.Estimator.EstimateFeePerKW(6)
 	if err != nil {
 		return nil, err
 	}
-	txFee := feePerVSize.FeeForVSize(txVSize)
+	txFee := feePerKw.FeeForWeight(txWeight)
 
 	// Sweep as much possible, after subtracting txn fees.
 	sweepAmt := int64(totalSum - txFee)