Add opt-in full-RBF to mempool

Replaces transactions already in the mempool if a new transaction seen
with a higher fee, specifically both a higher fee per KB and a higher
absolute fee. Children are evaluateed for replacement as well, using the
mempool package tracking to calculate replaced fees/size. Transactions
can opt-out of transaction replacement by setting nSequence >= maxint-1
on all inputs. (which all wallets do already)