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.
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.
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.
