Accept RBF bumps of single transactions (ie which conflict with one
transaction) even when that transaction is a member of a package
which is currently at the package limit iff the new transaction
does not add any additional mempool dependencies from the original.
This could be made a bit looser in the future and still be safe,
but for now this fixes the case that a transaction which was
accepted by the carve-out rule will not be directly RBF'able.
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
