Replaces transactions already in the mempool if a new transaction is
seen with a higher fee, provided that the replacement transaction's
outputs pay all previous outputs an equal or greater amount. This
preserves the "first seen" behavior of the mempool, in the sense that a
transaction will never be replaced in a fashion that prevents an address
from receiving funds that it otherwise would have. In short, zero-conf
transactions are unaffected.

To prevent replacements from being used as a DoS attack mechanism a
replacement only happens if the new transaction has a higher fee, pays a
higher fee/KB rate, and the fee increase is sufficient to pay for the
bandwidth consumed in relaying the replacement.

Includes stand-alone unittests for regtest in qa/replace-by-fee/