[SPARK-3623][GraphX] GraphX should support the checkpoint operation

Author: GuoQiang Li <witgo@qq.com>

Closes #2631 from witgo/SPARK-3623 and squashes the following commits:

a70c500 [GuoQiang Li] Remove java related
4d1e249 [GuoQiang Li] Add comments
e682724 [GuoQiang Li] Graph should support the checkpoint operation
[SPARK-4646] Replace Scala.util.Sorting.quickSort with Sorter(TimSort) in Spark

This patch just replaces a native quick sorter with Sorter(TimSort) in Spark.
It could get performance gains by ~8% in my quick experiments.

Author: Takeshi Yamamuro <linguin.m.s@gmail.com>

Closes #3507 from maropu/TimSortInEdgePartitionBuilderSpike and squashes the following commits:

8d4e5d2 [Takeshi Yamamuro] Remove a wildcard import
3527e00 [Takeshi Yamamuro] Replace Scala.util.Sorting.quickSort with Sorter(TimSort) in Spark
[SPARK-4620] Add unpersist in Graph and GraphImpl

Add an IF to uncache both vertices and edges of Graph/GraphImpl.
This IF is useful when iterative graph operations build a new graph in each iteration, and the vertices and edges of previous iterations are no longer needed for following iterations.

Author: Takeshi Yamamuro <linguin.m.s@gmail.com>

This patch had conflicts when merged, resolved by
Committer: Ankur Dave <ankurdave@gmail.com>

Closes #3476 from maropu/UnpersistInGraphSpike and squashes the following commits:

77a006a [Takeshi Yamamuro] Add unpersist in Graph and GraphImpl