Add simple replace-by-fee logic to mempool.

Replaces transactions if a new transaction is seen with a higher fee,
specifically both a higher fee per KB and a higher absolute fee.

Note that because the total fee is not computed recursively there exists
a simple DoS attack: create a long chain, paying for network bandwidth
with fees, and then replace the whole chain in one go with a single,
slightly higher fee tx.

This vulnerability is intentional to discourage non-testnet use of this
patch for now.