sweep: add new inteface InputSet to manage inputs in a tx

Previously the fee rate is tracked at cluster level, which may not be
accurate as each cluster is then divided into input sets. And these sets
are what's actually included in the final sweeping tx. To properly
reflect the final fee rate used in the sweeping tx, `InputSet` is added
so more customized clustering logic can be implemented in the future.
For intance, atm it's clustered by fee rates, while we may also cluster
by deadlines, urgencies, etc.

sweep/txgenerator.go

@@ -30,22 +30,15 @@ var (
 	ErrLocktimeConflict = errors.New("incompatible locktime")
 )
 
-// txInput is an interface that provides the input data required for tx
-// generation.
-type txInput interface {
-	input.Input
-	parameters() Params
-}
-
 // createSweepTx builds a signed tx spending the inputs to the given outputs,
 // sending any leftover change to the change script.
 func createSweepTx(inputs []input.Input, outputs []*wire.TxOut,
 	changePkScript []byte, currentBlockHeight uint32,
-	feePerKw, maxFeeRate chainfee.SatPerKWeight,
+	feeRate, maxFeeRate chainfee.SatPerKWeight,
 	signer input.Signer) (*wire.MsgTx, btcutil.Amount, error) {
 
 	inputs, estimator, err := getWeightEstimate(
-		inputs, outputs, feePerKw, maxFeeRate, changePkScript,
+		inputs, outputs, feeRate, maxFeeRate, changePkScript,
 	)
 	if err != nil {
 		return nil, 0, err
@@ -216,7 +209,7 @@ func createSweepTx(inputs []input.Input, outputs []*wire.TxOut,
 		"using %v sat/kw, tx_weight=%v, tx_fee=%v, parents_count=%v, "+
 		"parents_fee=%v, parents_weight=%v",
 		sweepTx.TxHash(), len(inputs),
-		inputTypeSummary(inputs), int64(feePerKw),
+		inputTypeSummary(inputs), feeRate,
 		estimator.weight(), txFee,
 		len(estimator.parents), estimator.parentsFee,
 		estimator.parentsWeight,
@@ -229,8 +222,7 @@ func createSweepTx(inputs []input.Input, outputs []*wire.TxOut,
 // Additionally, it returns counts for the number of csv and cltv inputs.
 func getWeightEstimate(inputs []input.Input, outputs []*wire.TxOut,
 	feeRate, maxFeeRate chainfee.SatPerKWeight,
-	outputPkScript []byte) ([]input.Input,
-	*weightEstimator, error) {
+	outputPkScript []byte) ([]input.Input, *weightEstimator, error) {
 
 	// We initialize a weight estimator so we can accurately asses the
 	// amount of fees we need to pay for this sweep transaction.