Add DFS traversal option to Repeat step

See discussion: #3777

Currently JanusGraph supports only TinkerPop's default Repeat step implementation which is using Breadth-First Search instead of Depth-Fist Search. I believe Depth-Fist Search might be a better solution to number of use cases, knowing that we already support query batching.
I suppose that batched DFS repeat steps might be a better default implementation for repeat step.

Start by reading the discussion in #3777 and the current TinkerPop Repeat step implementation referenced in the issue. Clarify whether the goal is an optional DFS traversal or a new default, then define the affected traversal behavior and tests before implementation.

Written by the indexing model from the issue text.