[SPARK-9418][SQL] Use sort-merge join as the default shuffle join.

Sort-merge join is more robust in Spark since sorting can be made using the Tungsten sort operator.

Author: Reynold Xin <rxin@databricks.com>

Closes #7733 from rxin/smj and squashes the following commits:

61e4d34 [Reynold Xin] Fixed test case.
5ffd731 [Reynold Xin] Fixed JoinSuite.
a137dc0 [Reynold Xin] [SPARK-9418][SQL] Use sort-merge join as the default shuffle join.