Make GraphReader and GraphWriter more friendly to GraphTraversalSource

We've been migrating users further and further away from the Graph API. The recently added io() step helps with that immensely. In that process, the GraphReader and GraphWriter interfaces probably should have went a step further to include an overloads for readGraph() and writeGraph() that used a GraphTraversalSource with deprecation for the Graph versions. Again, we don't need to remove these methods any time soon, but the preference should be toward preferring GraphTraversalSource when possible. This can be implemented as a non-breaking change to the API for those who have their own implementations of these interfaces.