Test protobuf <-> datomic roundtrips

[dgrnbrg]

This is meant to test that we can submit some JSON through the rest api, see it hit Datomic, then convert that to a protobuf, then follow the whole roundtrip back. This could catch potentially unknown serialization/format munging bugs, since we represent job data as Clojure datastructures, Mesos protobufs, Datomic datoms, and JSON objects.