"Quick and dirty" isDoubleSpend() check evaluates to false if the
conflicting tx has opted in to RBF. More verbose checks are still
required for BIP125. General RBF tests added.

Implements and tests BIP125 rules specifically pertaining to
fee requirements for replacement transactions. Valid replacements
must pay a higher fee rate than the original TX, and also must pay for
the bandwidth of all potentially evicted transactions in addition
to its own bandwidth.

Implements two extra rules defined in BIP125:
  - The replacement transaction may only include an unconfirmed input
  if that input was included in one of the original transactions.

  - The number of original transactions to be replaced and their
  descendant transactions which will be evicted from the mempool must
  not exceed a total of 100 transactions.

All the BIP125 rules have been implemented to reject invalid
replacement TXs. This is where the actual replacement happens,
adding the replacement TX and evicting all the conflicts and their
descendants.