[SPARK-4672][GraphX]Perform checkpoint() on PartitionsRDD to shorten the lineage

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

Iterative GraphX applications always have long lineage, while checkpoint() on EdgeRDD and VertexRDD themselves cannot shorten the lineage. In contrast, if we perform checkpoint() on their ParitionsRDD, the long lineage can be cut off. Moreover, the existing operations such as cache() in this code is performed on the PartitionsRDD, so checkpoint() should do the same way. More details and explanation can be found in the JIRA.

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

Closes #3549 from JerryLead/my_graphX_checkpoint and squashes the following commits:

d1aa8d8 [JerryLead] Perform checkpoint() on PartitionsRDD not VertexRDD and EdgeRDD themselves
ff08ed4 [JerryLead] Merge branch 'master' of https://github.com/apache/spark
c0169da [JerryLead] Merge branch 'master' of https://github.com/apache/spark
52799e3 [Lijie Xu] Merge pull request #1 from apache/master