OAK-842 : Incorrect interaction of orderable child nodes with permission evaluation
Partial undoing changes from r1565319: AbstractTree#getChildren need to call exists() in order to fixup potential missing moves as otherwise the call could return stale information. Also avoid casting by checking earlier for existence.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1565361 13f79535-47bb-0310-9956-ffa450edef68