[sql]use SparkSQLEnv.stop() in ShutdownHook

Author: wangfei <wangfei1@huawei.com>

Closes #1852 from scwf/patch-3 and squashes the following commits:

ae28c29 [wangfei] use SparkSQLEnv.stop() in ShutdownHook

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/HiveThriftServer2.scala

@@ -60,7 +60,7 @@ private[hive] object HiveThriftServer2 extends Logging {
     Runtime.getRuntime.addShutdownHook(
       new Thread() {
         override def run() {
-          SparkSQLEnv.sparkContext.stop()
+          SparkSQLEnv.stop()
         }
       }
     )