[SPARK-2409] Make SQLConf thread safe.

Author: Reynold Xin <rxin@apache.org>

Closes #1334 from rxin/sqlConfThreadSafetuy and squashes the following commits:

c1e0a5a [Reynold Xin] Fixed the duplicate comment.
7614372 [Reynold Xin] [SPARK-2409] Make SQLConf thread safe.

(cherry picked from commit 32516f866a32d51bfaa04685ae77ba216b4202d9)
Signed-off-by: Reynold Xin <rxin@apache.org>