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

The related JIRA is https://issues.apache.org/jira/browse/SPARK-4672

In a nutshell, if `val partitionsRDD` of VertexRDD and EdgeRDD are non-transient, the task's serialization chain will become very long in iterative algorithms and finally lead to the StackOverflow error. More details and explanation can be found in the JIRA.