OAK-850: Degrade gracefully when :childOrder is out of sync

Optimize the ordered getChildNames() operation for the common case where the child order list is in sync with the actual child nodes

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1579360 13f79535-47bb-0310-9956-ffa450edef68