[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

(cherry picked from commit 2e6b736b0e6e5920d0523533c87832a53211db42)
Signed-off-by: Ankur Dave <ankurdave@gmail.com>
[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

(cherry picked from commit 8817fc7fe8785d7b11138ca744f22f7e70f1f0a0)
Signed-off-by: Ankur Dave <ankurdave@gmail.com>