[SQL] Make functionRegistry in HiveContext transient.

Seems we missed `transient` for the `functionRegistry` in `HiveContext`.

cc: marmbrus

Author: Yin Huai <huaiyin.thu@gmail.com>

Closes #2074 from yhuai/makeFunctionRegistryTransient and squashes the following commits:

6534e7d [Yin Huai] Make functionRegistry transient.

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveContext.scala

@@ -255,6 +255,7 @@ class HiveContext(sc: SparkContext) extends SQLContext(sc) {
   }
 
   // Note that HiveUDFs will be overridden by functions registered in this context.
+  @transient
   override protected[sql] lazy val functionRegistry =
     new HiveFunctionRegistry with OverrideFunctionRegistry