(PDB-489) return producer timestamp

This exposes our "producer-timestamp" field on the factsets and catalogs
endpoints.  On factsets the field is queryable and orderable. The next step
will be to push the generation of this field back to the agent.
@senior thought about that -- it would fix the problem.  Our thought was that it would cause inconsistency other hyphenated responses?  How do we feel about going full-underscore?