Make proactive serializability checking optional.

SparkContext.clean uses ClosureCleaner's proactive serializability
checking by default.  This commit adds a parameter to the clean method
of SparkContext that allows clients to specify that serializability
checking should not occur as part of closure cleaning.

Author: willb

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -1205,8 +1205,8 @@ class SparkContext(config: SparkConf) extends Logging {
    * Clean a closure to make it ready to serialized and send to tasks
    * (removes unreferenced variables in $outer's, updates REPL variables)
    */
-  private[spark] def clean[F <: AnyRef](f: F): F = {
-    ClosureCleaner.clean(f)
+  private[spark] def clean[F <: AnyRef](f: F, checkSerializable: Boolean = true): F = {
+    ClosureCleaner.clean(f, checkSerializable)
     f
   }