sweep: wrap weight estimator

Preparation for a cpfp-aware weight estimator. For cpfp, a regular
weight estimator isn't sufficient, because it needs to take into account
the weight of the parent transaction(s) as well.

sweep/weight_estimator.go

@@ -0,0 +1,40 @@
+package sweep
+
+import (
+	"github.com/lightningnetwork/lnd/input"
+)
+
+// weightEstimator wraps a standard weight estimator instance.
+type weightEstimator struct {
+	estimator input.TxWeightEstimator
+}
+
+// newWeightEstimator instantiates a new sweeper weight estimator.
+func newWeightEstimator() *weightEstimator {
+	return &weightEstimator{}
+}
+
+// clone returns a copy of this weight estimator.
+func (w *weightEstimator) clone() *weightEstimator {
+	return &weightEstimator{
+		estimator: w.estimator,
+	}
+}
+
+// add adds the weight of the given input to the weight estimate.
+func (w *weightEstimator) add(inp input.Input) error {
+	wt := inp.WitnessType()
+
+	return wt.AddWeightEstimation(&w.estimator)
+}
+
+// addP2WKHOutput updates the weight estimate to account for an additional
+// native P2WKH output.
+func (w *weightEstimator) addP2WKHOutput() {
+	w.estimator.AddP2WKHOutput()
+}
+
+// weight gets the estimated weight of the transaction.
+func (w *weightEstimator) weight() int {
+	return w.estimator.Weight()
+}