OAK-672: Avoid JCR APIs calling other JCR APIs
OAK-662: Reduce boilerplate code in JCR impl methods
 - move common utility methods to ItemImpl
 - wrap implementation into ItemOperation where necessary. Except for PropertyImpl.setValue(s) methods, which I'll handle separately

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1461230 13f79535-47bb-0310-9956-ffa450edef68