"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.

We only need to call the "quick and dirty test" if the user has
set mempool RBF option to false. Do the Rule #1 check in verifyRBF()
instead where it belongs. This leaves one loose end in maybeOrphan()
that must be caught so we don't treat double-spends like orphans.

The current implementation improperly assumed that any unconfirmed
inputs spent by the replacement TX were also spent by its direct
conflict. It therefore did not account for the case where the
replacement was spending an unconfirmed CHILD of its direct
replacement. This would cause it to replace its own inputs which
is an invalid mempool state and led to unexpected errors.