OAK-796 : PermissionValidator: proper check for jcr:uuid modifications. 

- omit permission check if jcr:uuid is defined by mix:referenceable
- execute default permission checks if the defining node is not mix:referenceable

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1482783 13f79535-47bb-0310-9956-ffa450edef68