Handle versioning of primary datasets

[shirey]

Currently datasets, on POST/create, can specify a previous_revision_uuid which creates a REVISION_OF relationship in Neo4j to an older version of a dataset. We have only ever done this for processed/derived (has another dataset as a parent) dataset. We need to start doing this for primary datasets too (has a Sample as a parent). To this end we need to make sure that:

• we aren't constraining this feature to derived datasets at POST time
• any query that returns "provenance information" will handle these correctly
• the next_revision_uuid and previous_revision_uuid fields are generated correctly on a GET call