Support for MessageScope.Subgraph in GraphComputer

I don't really know the best way to express this, but I think we should allow a VertexProgram to state which vertices it should process at each iteration. For most VertexProgram}}s only those vertices that received a message in the last iteration need to be evaluated. What is the best way to say this so that {{GraphComputer implementations can use it, I don't know.

This is all really related to GraphFilter and MessageScope as well. Basically, at every iteration, what subgraph do you need to grab to process? ... we should really tie all those concepts together nicely.