OAK-2151: DocumentNodeStore does not detect hierarchy conflict

Improved test and changed the commit logic to always set the _children field on the parent of an added node, even if it is already present. This materializes the conflict and result in a merge failure for one of the commits. 

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1628447 13f79535-47bb-0310-9956-ffa450edef68