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/sweeper.go

@@ -760,7 +760,7 @@ func (s *UtxoSweeper) sweepCluster(cluster Cluster) error {
 
 		// Create sweeping transaction for each set.
 		for _, inputs := range sets {
-			err := s.sweep(inputs, cluster.FeeRate())
+			err := s.sweep(inputs)
 			if err != nil {
 				log.Errorf("sweep new inputs: %w", err)
 			}
@@ -803,9 +803,7 @@ func (s *UtxoSweeper) signalResult(pi *pendingInput, result Result) {
 
 // sweep takes a set of preselected inputs, creates a sweep tx and publishes the
 // tx. The output address is only marked as used if the publish succeeds.
-func (s *UtxoSweeper) sweep(inputs inputSet,
-	feeRate chainfee.SatPerKWeight) error {
-
+func (s *UtxoSweeper) sweep(set InputSet) error {
 	// Generate an output script if there isn't an unused script available.
 	if s.currentOutputScript == nil {
 		pkScript, err := s.cfg.GenSweepScript()
@@ -817,8 +815,9 @@ func (s *UtxoSweeper) sweep(inputs inputSet,
 
 	// Create sweep tx.
 	tx, fee, err := createSweepTx(
-		inputs, nil, s.currentOutputScript, uint32(s.currentHeight),
-		feeRate, s.cfg.MaxFeeRate.FeePerKWeight(), s.cfg.Signer,
+		set.Inputs(), nil, s.currentOutputScript,
+		uint32(s.currentHeight), set.FeeRate(),
+		s.cfg.MaxFeeRate.FeePerKWeight(), s.cfg.Signer,
 	)
 	if err != nil {
 		return fmt.Errorf("create sweep tx: %w", err)
@@ -826,7 +825,7 @@ func (s *UtxoSweeper) sweep(inputs inputSet,
 
 	tr := &TxRecord{
 		Txid:    tx.TxHash(),
-		FeeRate: uint64(feeRate),
+		FeeRate: uint64(set.FeeRate()),
 		Fee:     uint64(fee),
 	}
 
@@ -1195,6 +1194,8 @@ func (s *UtxoSweeper) handleUpdateReq(req *updateReq) (
 // - Make handling re-orgs easier.
 // - Thwart future possible fee sniping attempts.
 // - Make us blend in with the bitcoind wallet.
+//
+// TODO(yy): remove this method and only allow sweeping via requests.
 func (s *UtxoSweeper) CreateSweepTx(inputs []input.Input,
 	feePref FeeEstimateInfo) (*wire.MsgTx, error) {