OAK-1807 - ConstraintViolationException seen with multiple Oak/Mongo with ConcurrentCreateNodesTest

Adding an ignored testcase which demonstrates that duplicate revision might get created with multiple threads

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1593245 13f79535-47bb-0310-9956-ffa450edef68