[SPARK-1508][SQL] Add SQLConf to SQLContext.

This is still work in progress. (Potentially incomplete) List of things to do:

* Support set commands for `hql()`.
* Hook this into HiveContext as well.
* Run an end-to-end test on a cluster.
I meant java.util.Collections.synchronizedMap

Don't use ConcurrentHashMap - it has terrible memory footprint and performance in low contention. In most cases, this will be accessed in a single thread.