[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

(cherry picked from commit 17c162f6682520e6e2790626e37da3a074471793)
Signed-off-by: Ankur Dave <ankurdave@gmail.com>