[SPARK-9109] [GRAPHX] Keep the cached edge in the graph

The change here is to keep the cached RDDs in the graph object so that when the graph.unpersist() is called these RDDs are correctly unpersisted.

```java
import org.apache.spark.graphx._
import org.apache.spark.rdd.RDD
import org.slf4j.LoggerFactory
import org.apache.spark.graphx.util.GraphGenerators

// Create an RDD for the vertices
val users: RDD[(VertexId, (String, String))] =
  sc.parallelize(Array((3L, ("rxin", "student")), (7L, ("jgonzal", "postdoc")),
                       (5L, ("franklin", "prof")), (2L, ("istoica", "prof"))))
// Create an RDD for edges
val relationships: RDD[Edge[String]] =
  sc.parallelize(Array(Edge(3L, 7L, "collab"),    Edge(5L, 3L, "advisor"),
                       Edge(2L, 5L, "colleague"), Edge(5L, 7L, "pi")))
// Define a default user in case there are relationship with missing user
val defaultUser = ("John Doe", "Missing")
// Build the initial Graph
val graph = Graph(users, relationships, defaultUser)
graph.cache().numEdges

graph.unpersist()

sc.getPersistentRDDs.foreach( r => println( r._2.toString))
```

Author: tien-dungle <tien-dung.le@realimpactanalytics.com>

Closes #7469 from tien-dungle/SPARK-9109_Graphx-unpersist and squashes the following commits:

8d87997 [tien-dungle] Keep the cached edge in the graph