Support runInBackground in SparkExecuteStatementOperation, and add cancellation

Author: Dong Wang <dong@databricks.com>

Closes #6207 from dongwang218/SPARK-6964-jdbc-cancel and squashes the following commits:

687c113 [Dong Wang] fix 100 characters
7bfa2a7 [Dong Wang] fix merge
380480f [Dong Wang] fix for liancheng's comments
eb3e385 [Dong Wang] small nit
341885b [Dong Wang] small fix
3d8ebf8 [Dong Wang] add spark.sql.hive.thriftServer.async flag
04142c3 [Dong Wang] set SQLSession for async execution
184ec35 [Dong Wang] keep hive conf
819ae03 [Dong Wang] [SPARK-6964][SQL][WIP] Support Cancellation in the Thrift Server