Support 'custom partitionKey and custom clusteringKey' as DSE Graph

DSE Graph supports feature 'Custom Vertex Id', actually it is 'Custom partitionKey and custom clusteringKey', as mentioned in the two links below. The feature is for graph partitioning.

1. DSE Graph Partitioning Part 1: Custom Vertex Ids, 2016
https://www.experoinc.com/post/dse-graph-partitioning-part-1-custom-vertex-ids

2. Using DSE Graph--Creating a custom vertex id
https://docs.datastax.com/en/dse/5.1/dse-dev/datastax_enterprise/graph/using/createCustVertexId.html

The feature 'custom partitionKey and custom clusteringKey' has less limitation than feature mentioned in http://docs.janusgraph.org/latest/graph-partitioning.html.

Can janusgraph support it?

Start by comparing the two linked DSE Graph references with JanusGraph's graph-partitioning documentation. Define the supported custom partitionKey and clusteringKey behavior, its limitations, and the scope of compatibility; the issue is complete only when that design and support boundary are agreed.

Written by the indexing model from the issue text.