don't export draft children of orphaned drafted (TNL-923)

@dmitchell 
@zubair-arbi 

Fixes an issue where the draft children of orphaned drafts were being exported

related bugs:
https://openedx.atlassian.net/browse/TNL-923
https://openedx.atlassian.net/browse/TNL-872
Ok, I haven't thought it thru, but do we run into the same issue with children under more than one subtree (DAG) some of which subtrees are orphans causing the children to get "deleted"?