(maint) jsonify benchmark submission objects

This is necessary for catalog and report submission to work with benchmark.clj.
Merge pull request #1114 from wkalt/maint/master/jsonify-benchmark-objects

(maint) jsonify benchmark submission objects
(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.