OAK-1584: Performance regression of adding and removinf child nodes after OAK-850

Don't scan the full list of child nodes whenever adding or removing children (to an orderable node)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1579637 13f79535-47bb-0310-9956-ffa450edef68