(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.
Merge pull request #1113 from wkalt/ticket/master/pdb-489-return-producer-timestamp

(PDB-489) return producer timestamp