Improve the memory usage in StronglyConnectedComponents

This is very consume memory when It use StronglyConnectedComponents(see figure1.png). Because there is no mark the Graph/RDD as non-persistent in the iterative process timely. And it is maybe lead to fail in the big graph.

In order to improve the memory usage, it is verty important to mark the Graph/RDD as non-persistent timely. In the current code, only make the Graph/RDD as non-persistent for 'sccGraph' but not for 'sccWorkGraph' in degree's step and pregel's step.

I have done a optimized code proposal(see my fork:https://github.com/jmzhoulab/spark/blob/master/graphx/src/main/scala/org/apache/spark/graphx/lib/StronglyConnectedComponents.scala)

The storage after optimization see figure2.png