[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 noticed that [mutable.SynchronizedMap](http://www.scala-lang.org/api/2.11.0/index.html#scala.collection.mutable.SynchronizedMap) is deprecated in Scala 2.11 -- do you mean [TrieMap](http://www.scala-lang.org/api/2.10.0/index.html#scala.collection.concurrent.TrieMap)?

As an alternative, we could also use a [java.util.concurrent.ConcurrentHashMap](http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html), although this class does not support nice-to-have funcs such as `getOrElse()` and `apply()`.