(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.
We should move most of this pre/post check functionality to schema when we can. The intersection test is probably 'okay' as a pre check however, since its more logically complex.