[SPARK-11089][SQL] Adds option for disabling multi-session in Thrift server

This PR adds a new option `spark.sql.hive.thriftServer.singleSession` for disabling multi-session support in the Thrift server.

Note that this option is added as a Spark configuration (retrieved from `SparkConf`) rather than Spark SQL configuration (retrieved from `SQLConf`). This is because all SQL configurations are session-ized. Since multi-session support is by default on, no JDBC connection can modify global configurations like the newly added one.

Author: Cheng Lian <lian@databricks.com>

Closes #9740 from liancheng/spark-11089.single-session-option.