[SQL] Pass SQLContext instead of SparkContext into physical operators.

This makes it easier to use config options in operators.

Author: Reynold Xin <rxin@apache.org>

Closes #1164 from rxin/sqlcontext and squashes the following commits:

797b2fd [Reynold Xin] Pass SQLContext instead of SparkContext into physical operators.

(cherry picked from commit ca5d8b5904dc6dd5b691af506d3a842e508b3673)
Signed-off-by: Reynold Xin <rxin@apache.org>