Support querying a specific dataset copy.

http://soda-fountain/resource/dataset?[$$copy=latest|published|unpublished]
Currently, the value for copy is "published", "unpublished" and "latest".  I am not sure how we want to handle snapshots vs copy number.  Anyway, qc is internal.  sf (soda fountain) probably should be where we put validation.  But I haven't done so in soda fountain because I don't see this being finalized.  I hope the work will trigger further discussion on the publication work flow.