Minimize use of jtx::with_only_features (RIPD-1515):

In order to automatically run unit tests with newly created
amendments, prefer to start with jtx::supported_features() and
then subtract unwanted features.

These changes identified a few bugs that were hiding in
amendments.  One of those bugs, in FlowCross, is not yet fixed.
By uncommenting the test in CrossingLimits_test.cpp you can see
failures relating to that bug.  Since FlowCross is not yet
enabled on the network we can fix the bug at our convenience.

src/ripple/protocol/impl/Feature.cpp

@@ -129,6 +129,12 @@ uint256 bitsetIndexToFeature(size_t i)
     return featureCollections.bitsetIndexToFeature(i);
 }
 
+bool hasFeature(uint256 const& feat, FeatureBitset features)
+{
+    return features[featureToBitsetIndex(feat)];
+}
+
+
 uint256 const featureMultiSign = *getRegisteredFeature("MultiSign");
 uint256 const featureTickets = *getRegisteredFeature("Tickets");
 uint256 const featureTrustSetAuth = *getRegisteredFeature("TrustSetAuth");