[SPARK-5355] make SparkConf thread-safe

The SparkConf is not thread-safe, but is accessed by many threads. The getAll() could return parts of the configs if another thread is access it.

This PR changes SparkConf.settings to a thread-safe TrieMap.

Author: Davies Liu <davies@databricks.com>

Closes #4143 from davies/safe-conf and squashes the following commits:

f8fa1cf [Davies Liu] change to TrieMap
a1d769a [Davies Liu] make SparkConf thread-safe