sweep/tx_input_set: account for required outputs in input set

If inputs require outputs to be added at the same time, this will
change the weight and amount calculations, so we must account for that.

We wait to get the weight estimator for the sweep tx until needed,
such that we can easily choose whether to include a change output or not
in the estimate. This is needed for the case where the second level
transactions can pay for their own fee, so no change output is needed.

sweep/txgenerator.go

@@ -110,17 +110,19 @@ func generateInputPartitionings(sweepableInputs []txInput,
 		// the dust limit, stop sweeping. Because of the sorting,
 		// continuing with the remaining inputs will only lead to sets
 		// with an even lower output value.
-		if !txInputs.dustLimitReached() {
-			log.Debugf("Set value %v below dust limit of %v",
-				txInputs.outputValue, txInputs.dustLimit)
+		if !txInputs.enoughInput() {
+			log.Debugf("Set value %v (r=%v, c=%v) below dust "+
+				"limit of %v", txInputs.totalOutput(),
+				txInputs.requiredOutput, txInputs.changeOutput,
+				txInputs.dustLimit)
 			return sets, nil
 		}
 
 		log.Infof("Candidate sweep set of size=%v (+%v wallet inputs), "+
 			"has yield=%v, weight=%v",
 			inputCount, len(txInputs.inputs)-inputCount,
-			txInputs.outputValue-txInputs.walletInputTotal,
-			txInputs.weightEstimate.weight())
+			txInputs.totalOutput()-txInputs.walletInputTotal,
+			txInputs.weightEstimate(true).weight())
 
 		sets = append(sets, txInputs.inputs)
 		sweepableInputs = sweepableInputs[inputCount:]