Greatly simplified TraversalVertexProgram. Now that Traversers have bulk, there is no need to wrap them into a TraverserMessage that has a bulk field. Now, the Traverser IS the message. Super simple. Moreover, made the processing of the Traversers simple -- code complexity was reduced significantly. Added JavaDoc to VertexProgram for @dalaro. Renamed MessageType.of() to MessageType.to() as you send message 'to' something and thus, more resaonable method name.