Remove ParentSpanID from domain model #831
This implements option 1 from #824 by removing ParentSpanID from the domain model and only using the References array.
Once unfortunate discovery: both Cassandra and Elasticsearch models kept the ParentID field in their respective models. It is no longer populated in the domain->db model conversions, but still has to be taken into account in the dbmodel->domain conversion for backwards compatibility with existing data. I don't know of a better solution.
A side effect of that is that we won't be able to use gogoprotobuf-generated model (from #773) to generate the JSON for Elasticsearch. The plan would be to move