[SPARK-4620] Add unpersist in Graph and GraphImpl

Add an IF to uncache both vertices and edges of Graph/GraphImpl.
This IF is useful when iterative graph operations build a new graph in each iteration, and the vertices and edges of previous iterations are no longer needed for following iterations.

Author: Takeshi Yamamuro <linguin.m.s@gmail.com>

This patch had conflicts when merged, resolved by
Committer: Ankur Dave <ankurdave@gmail.com>

Closes #3476 from maropu/UnpersistInGraphSpike and squashes the following commits:

77a006a [Takeshi Yamamuro] Add unpersist in Graph and GraphImpl

graphx/src/main/scala/org/apache/spark/graphx/Graph.scala

@@ -104,6 +104,12 @@ abstract class Graph[VD: ClassTag, ED: ClassTag] protected () extends Serializab
    */
   def checkpoint(): Unit
 
+  /**
+   * Uncaches both vertices and edges of this graph. This is useful in iterative algorithms that
+   * build a new graph in each iteration.
+   */
+  def unpersist(blocking: Boolean = true): Graph[VD, ED]
+
   /**
    * Uncaches only the vertices of this graph, leaving the edges alone. This is useful in iterative
    * algorithms that modify the vertex attributes but reuse the edges. This method can be used to

graphx/src/main/scala/org/apache/spark/graphx/impl/GraphImpl.scala

@@ -70,6 +70,12 @@ class GraphImpl[VD: ClassTag, ED: ClassTag] protected (
     replicatedVertexView.edges.checkpoint()
   }
 
+  override def unpersist(blocking: Boolean = true): Graph[VD, ED] = {
+    unpersistVertices(blocking)
+    replicatedVertexView.edges.unpersist(blocking)
+    this
+  }
+
   override def unpersistVertices(blocking: Boolean = true): Graph[VD, ED] = {
     vertices.unpersist(blocking)
     // TODO: unpersist the replicated vertices in `replicatedVertexView` but leave the edges alone