OAK-1186: Parallel execution of ConcurrentReadAccessControlledTreeTest fails with MongoMK
- change MongoMK.reset() to actually undo the commits, applying the reverse diff is not sufficient
- additional tests

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1545608 13f79535-47bb-0310-9956-ffa450edef68