Allow a Traversal to know what TraversalSource it spawned from

It's odd that a Traversal has no knowledge of the TraversalSource it spawned from. I say "odd" because:

1. we pass the TraversalSource to the Traversal on construction only to extract information from it and then throw it away
2. one of the data points we do grab is the Graph instance of the TraversalSource and for some reason we keep that around for the purpose of sometime spawning more TraversalSource instances in other components.

I'm not sure if there was a reason for not keeping the TraversalSource at some point, but I'm going to add it and see what happens.