OAK-928: Read access is enforced on NEW items

Make the special nature of transient changes more obvious:
It's possible to commit content that you only have write access to;
and in that case the content would appear to disappear after the commit!

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1545807 13f79535-47bb-0310-9956-ffa450edef68