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