Make proactive serializability checking optional.

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

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

@@ -1032,7 +1032,15 @@ class SparkContext(
    * (removes unreferenced variables in $outer's, updates REPL variables)
    */
   private[spark] def clean[F <: AnyRef](f: F): F = {
-    ClosureCleaner.clean(f)
+    clean(f, true)
+  }
+
+  /**
+   * 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, checkSerializable: Boolean): F = {
+    ClosureCleaner.clean(f, checkSerializable)
     f
   }