HIVE-7707 : Optimize SparkMapRecordHandler implementation (Chengxiang Li via Szehon) [Spark Branch]

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/spark@1618002 13f79535-47bb-0310-9956-ffa450edef68
Cleanup SparkClient and make refreshLocalResources method synchronized.