[SPARK-4594][SQL] Improvement the broadcast for HiveConf

https://issues.apache.org/jira/browse/SPARK-4594
Every time we need to get a table from hive , HadoopTableReader will broadcast HiveConf to clustor . Acturally In one application the hiveconf is single, so I think we can keep it in HiveContext for every query . Although it just 50kb , it's useful for JDBC user and streaming+sql app .