Instantiate SparkClient per user session [Spark Branch]

SparkContext is the main class via which Hive talk to Spark cluster. SparkClient encapsulates a SparkContext instance. Currently all user sessions share a single SparkClient instance in HiveServer2. While this is good enough for a POC, even for our first two milestones, this is not desirable for a multi-tenancy environment and gives least flexibility to Hive users. Here is what we propose:

1. Have a SparkClient instance per user session. The SparkClient instance is created when user executes its first query in the session. It will get destroyed when user session ends.

2. The SparkClient is instantiated based on the spark configurations that are available to the user, including those defined at the global level and those overwritten by the user (thru set command, for instance).

3. Ideally, when user changes any spark configuration during the session, the old SparkClient instance should be destroyed and a new one based on the new configurations is created. This may turn out to be a little hard, and thus it's a "nice-to-have". If not implemented, we need to document that subsequent configuration changes will not take effect in the current session.

Please note that there is a thread-safety issue on Spark side where multiple SparkContext instances cannot coexist in the same JVM (SPARK-2243). We need to work with Spark community to get this addressed.

Besides above functional requirements, avoid potential issues is also a consideration. For instance, sharing SC among users is bad, as resources (such as jar for UDF) will be also shared, which is problematic. On the other hand, one SC per job seems too expensive, as the resource needs to be re-rendered even there isn't any change.