[SQL] add note of use synchronizedMap in SQLConf

Refer to:
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.

Author: wangfei <wangfei_hello@126.com>
Author: scwf <wangfei1@huawei.com>

Closes #1996 from scwf/sqlconf and squashes the following commits:

93bc0c5 [wangfei] revert change of HiveQuerySuite
0cc05dd [wangfei] add note for use synchronizedMap
3c224d31 [scwf] fix formate
a7bcb98 [scwf] use ConcurrentHashMap in sql conf, intead synchronizedMap

sql/core/src/main/scala/org/apache/spark/sql/SQLConf.scala

@@ -54,6 +54,7 @@ private[spark] object SQLConf {
 trait SQLConf {
   import SQLConf._
 
+  /** Only low degree of contention is expected for conf, thus NOT using ConcurrentHashMap. */
   @transient protected[spark] val settings = java.util.Collections.synchronizedMap(
     new java.util.HashMap[String, String]())