[SPARK-5355] use j.u.c.ConcurrentHashMap instead of TrieMap

j.u.c.ConcurrentHashMap is more battle tested.

cc rxin JoshRosen pwendell

Author: Davies Liu <davies@databricks.com>

Closes #4208 from davies/safe-conf and squashes the following commits:

c2182dc [Davies Liu] address comments, fix tests
3a1d821 [Davies Liu] fix test
da14ced [Davies Liu] Merge branch 'master' of github.com:apache/spark into safe-conf
ae4d305 [Davies Liu] change to j.u.c.ConcurrentMap
f8fa1cf [Davies Liu] change to TrieMap
a1d769a [Davies Liu] make SparkConf thread-safe