sweep+cnct+nursery+rpc: extract DetermineFeePerKw to func, add FeePreference

In this commit, we extract the existing determineFeePerKw method on the
RPC server into a new file in the sweep package. Along the way, we
consolidate code by introducing a new FeePreference struct, which allows
the caller to express their fee preference either in blocks to
confirmation, or a direct fee rate. This move takes a small step to
father decoupling calls in the main RPC server.

sweep/sweeper.go

@@ -750,10 +750,10 @@ func (s *UtxoSweeper) waitForSpend(outpoint wire.OutPoint,
 // - Make handling re-orgs easier.
 // - Thwart future possible fee sniping attempts.
 // - Make us blend in with the bitcoind wallet.
-func (s *UtxoSweeper) CreateSweepTx(inputs []Input, confTarget uint32,
+func (s *UtxoSweeper) CreateSweepTx(inputs []Input, feePref FeePreference,
 	currentBlockHeight uint32) (*wire.MsgTx, error) {
 
-	feePerKw, err := s.cfg.FeeEstimator.EstimateFeePerKW(confTarget)
+	feePerKw, err := DetermineFeePerKw(s.cfg.FeeEstimator, feePref)
 	if err != nil {
 		return nil, err
 	}

sweep/walletsweep.go

@@ -0,0 +1,68 @@
+package sweep
+
+import (
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/lnwallet"
+)
+
+// FeePreference allows callers to express their time value for inclusion of a
+// transaction into a block via either a confirmation target, or a fee rate.
+type FeePreference struct {
+	// ConfTarget if non-zero, signals a fee preference expressed in the
+	// number of desired blocks between first broadcast, and confirmation.
+	ConfTarget uint32
+
+	// FeeRate if non-zero, signals a fee pre fence expressed in the fee
+	// rate expressed in sat/kw for a particular transaction.
+	FeeRate lnwallet.SatPerKWeight
+}
+
+// DetermineFeePerKw will determine the fee in sat/kw that should be paid given
+// an estimator, a confirmation target, and a manual value for sat/byte. A
+// value is chosen based on the two free parameters as one, or both of them can
+// be zero.
+func DetermineFeePerKw(feeEstimator lnwallet.FeeEstimator,
+	feePref FeePreference) (lnwallet.SatPerKWeight, error) {
+
+	switch {
+	// If the target number of confirmations is set, then we'll use that to
+	// consult our fee estimator for an adequate fee.
+	case feePref.ConfTarget != 0:
+		feePerKw, err := feeEstimator.EstimateFeePerKW(
+			uint32(feePref.ConfTarget),
+		)
+		if err != nil {
+			return 0, fmt.Errorf("unable to query fee "+
+				"estimator: %v", err)
+		}
+
+		return feePerKw, nil
+
+	// If a manual sat/byte fee rate is set, then we'll use that directly.
+	// We'll need to convert it to sat/kw as this is what we use
+	// internally.
+	case feePref.FeeRate != 0:
+		feePerKW := feePref.FeeRate
+		if feePerKW < lnwallet.FeePerKwFloor {
+			log.Infof("Manual fee rate input of %d sat/kw is "+
+				"too low, using %d sat/kw instead", feePerKW,
+				lnwallet.FeePerKwFloor)
+
+			feePerKW = lnwallet.FeePerKwFloor
+		}
+
+		return feePerKW, nil
+
+	// Otherwise, we'll attempt a relaxed confirmation target for the
+	// transaction
+	default:
+		feePerKw, err := feeEstimator.EstimateFeePerKW(6)
+		if err != nil {
+			return 0, fmt.Errorf("unable to query fee estimator: "+
+				"%v", err)
+		}
+
+		return feePerKw, nil
+	}
+}