apache/spark — pull request #1996 by scwf (2014-08-17 05:36:37+00:00)
Title: [SQL] use ConcurrentHashMap in SQLConf

http://stackoverflow.com/questions/510632/whats-the-difference-between-concurrenthashmap-and-collections-synchronizedmap
Collections.synchronizedMap(map) creates a blocking Map which will degrade performance, albeit ensure consistency. So use ConcurrentHashMap(a more effective thread-safe hashmap) instead.

also update HiveQuerySuite to fix test error when changed to ConcurrentHashMap.