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

Testcase which mimics multiple cluster node writes as done by running multiple instances of ConcurrentCreateNodesTest

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1593188 13f79535-47bb-0310-9956-ffa450edef68