Conservatively accept RBF bumps bumping one tx at the package limits
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.