OAK-1934: Optimize MutableTree.orderBefore for the common case

Only scan the actual child node list when the target node is not found in
the child order list or when appending to the end of the list. Ohterwise
use the already existing child order property.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1607152 13f79535-47bb-0310-9956-ffa450edef68