HIVE-7746 - Cleanup SparkClient and make refreshLocalResources method synchronized (Venki Korukanti via Brock) [Spark Branch]

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/spark@1618530 13f79535-47bb-0310-9956-ffa450edef68
HIVE-7766 - Cleanup Reduce operator code [Spark Branch] (Rui Li via Brock)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/spark@1618642 13f79535-47bb-0310-9956-ffa450edef68
HIVE-7763 - Failed to qeury TABLESAMPLE on empty bucket table [Spark Branch] (Chengxiang via Brock)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/spark@1618646 13f79535-47bb-0310-9956-ffa450edef68
HIVE-7606: Design SparkSession and SparkSessionManager.