Use payment flow code for offer crossing (RIPD-1094):

Replace Taker.cpp with calls to the payment flow() code.

This change required a number of tweaks in the payment flow code.
These tweaks are conditionalized on whether or not offer crossing
is taking place.  The flag is explicitly passed as a parameter to
the flow code.

For testing, a class was added that identifies differences in the
contents of two PaymentSandboxes.  That code may be reusable in
the future.

None of the Taker offer crossing code is removed.  Both versions
of the code are co-resident to support an amendment cut-over.

The code that identifies differences between Taker and Flow offer
crossing is enabled by a feature.  That makes it easy to enable
or disable difference logging by changing the config file.  This
approach models what was done with the payment flow code.  The
differencing code should never be enabled on a production server.

Extensive offer crossing unit tests are added to examine and
verify the behavior of corner cases.  The tests are currently
configured to run against both Taker and Flow offer crossing.
This gives us confidence that most cases run identically and
some of the (few) differences in behavior are documented.

src/ripple/protocol/impl/Feature.cpp

@@ -54,6 +54,8 @@ uint256 const featureCompareFlowV1V2 = feature("CompareFlowV1V2");
 uint256 const featureSHAMapV2 = feature("SHAMapV2");
 uint256 const featurePayChan = feature("PayChan");
 uint256 const featureFlow = feature("Flow");
+uint256 const featureCompareTakerFlowCross = feature("CompareTakerFlowCross");
+uint256 const featureFlowCross = feature("FlowCross");
 uint256 const featureCryptoConditions = feature("CryptoConditions");
 uint256 const featureTickSize = feature("TickSize");
 uint256 const fix1368 = feature("fix1368");