[SPARK-4672][GraphX]Non-transient PartitionsRDDs will lead to StackOverflow error

The related JIRA is https://issues.apache.org/jira/browse/SPARK-4672

In a nutshell, if `val partitionsRDD` in EdgeRDDImpl and VertexRDDImpl are non-transient, the serialization chain can become very long in iterative algorithms and finally lead to the StackOverflow error. More details and explanation can be found in the JIRA.

Author: JerryLead <JerryLead@163.com>
Author: Lijie Xu <csxulijie@gmail.com>

Closes #3544 from JerryLead/my_graphX and squashes the following commits:

628f33c [JerryLead] set PartitionsRDD to be transient in EdgeRDDImpl and VertexRDDImpl
c0169da [JerryLead] Merge branch 'master' of https://github.com/apache/spark
52799e3 [Lijie Xu] Merge pull request #1 from apache/master

Author: JerryLead

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

@@ -26,7 +26,7 @@ import org.apache.spark.storage.StorageLevel
 import org.apache.spark.graphx._
 
 class EdgeRDDImpl[ED: ClassTag, VD: ClassTag] private[graphx] (
-    override val partitionsRDD: RDD[(PartitionID, EdgePartition[ED, VD])],
+    @transient override val partitionsRDD: RDD[(PartitionID, EdgePartition[ED, VD])],
     val targetStorageLevel: StorageLevel = StorageLevel.MEMORY_ONLY)
   extends EdgeRDD[ED](partitionsRDD.context, List(new OneToOneDependency(partitionsRDD))) {
 

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

@@ -27,7 +27,7 @@ import org.apache.spark.storage.StorageLevel
 import org.apache.spark.graphx._
 
 class VertexRDDImpl[VD] private[graphx] (
-    val partitionsRDD: RDD[ShippableVertexPartition[VD]],
+    @transient val partitionsRDD: RDD[ShippableVertexPartition[VD]],
     val targetStorageLevel: StorageLevel = StorageLevel.MEMORY_ONLY)
   (implicit override protected val vdTag: ClassTag[VD])
   extends VertexRDD[VD](partitionsRDD.context, List(new OneToOneDependency(partitionsRDD))) {