I am a little confused about the relationship between JanusGraph and gremlin, the storage backend(HBase, Cassandra, .etc) is configured by JanusGraph, but and the core traversal strategy(GraphTraversal, GraphTraversalSource, .etc) are defined in gremlin, so how did janusgraph interact with the storage backend now that its core calculate engine(which is gremlin) knows nothing about the storage at all? Can you help me to know about what gremlin did in janusgraph, and what janusgraph did based on gremlin? Thanks in advance.