sweep: make sweeper aware of unconfirmed parent transactions.

Extend the fee estimator to take into account parent transactions with
their weights and fees.

Do not try to cpfp parent transactions that have a higher fee rate than
the sweep tx fee rate.

breacharbiter.go

@@ -914,6 +914,11 @@ func (bo *breachedOutput) HeightHint() uint32 {
 	return bo.confHeight
 }
 
+// UnconfParent returns information about a possibly unconfirmed parent tx.
+func (bo *breachedOutput) UnconfParent() *input.TxInfo {
+	return nil
+}
+
 // Add compile-time constraint ensuring breachedOutput implements the Input
 // interface.
 var _ input.Input = (*breachedOutput)(nil)