OAK-1159: Backup and restore

Add a test for the super-root comparison case
Fix the comparison to always use the root (and not super-root) state
Address the todo for optimizing the incremental backup

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1547796 13f79535-47bb-0310-9956-ffa450edef68