Prefer VertexRDD over VertexRDDImpl

[benmccann]

No description provided.

[bhoppi]

Thank you, but I prefer a full solution that hides EdgeRDDImpl and VertexRDDImpl at the same time, otherwise keep both of them.
But EdgeRDDImpl is hard to optimize by only using provided EdgeRDD APIs.

[benmccann]

Yes, EdgeRDDImpl is harder to hide, which is why I left it. But also because you only need to hide VertexRDDImpl in order to use the upstream graphx implementation, so you will get most of the benefit from just hiding VertexRDDImpl whereas hiding EdgeRDDImpl doesn't accomplish much.