This implements the proposed policy change from [1], which allows
certain classes of contract protocols involving revocation
punishments to use CPFP. Note that some such use-cases may still
want some form of one-deep package relay, though even this alone
may greatly simplify some lightning fee negotiation.
[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/016518.html
