Entity Graph over Persistent Graph DB

Currently, the Entity Graph is stored using the Python NetworkX package and in-memory. In this BP, we would like to additionally support holding the Entity Graph in a persistent graph DB (possible candidates - Titan, Neo4J), so that data is not lost during Virage crashes and also for performance gains. This change must also include support for multi-process & multi-threaded access to the Entity Graph, which is not possible in the current in-memory implementation.