PM-150 Added a new iterator that allows us to traverse the DAG bottom
in a BFS ordder.
PM-150 clustered job updated to support the new iteration function.
PM-150 updated the RemoveDirectory to add leaf cleanup jobs that only
remove the directory. The strategy employed to determine the edges
between the leaf cleanup is similar to the Minimal implementation for
adding the create dir jobs.
PM-150 for the time being leaf cleanup nodes are added whenever
cleanup is enabled.
PM-150 disable the removal of staged worker package as a separate
cleanup job. the leaf cleanup should take care of this.