[SPARK-5470][Core]use defaultClassLoader to load classes in KryoSerializer

Now KryoSerializer load classes of classesToRegister at the time of its initialization. when we set spark.kryo.classesToRegister=class1, it will throw SparkException("Failed to load class to register with Kryo".
because in KryoSerializer's initialization, classLoader cannot include class of user's jars.
we need to use defaultClassLoader of Serializer in newKryo(), because executor will reset defaultClassLoader of Serializer after Serializer's initialization.
thank zzcclp for reporting it to me.

Author: lianhuiwang <lianhuiwang09@gmail.com>

Closes #4258 from lianhuiwang/SPARK-5470 and squashes the following commits:

73b719f [lianhuiwang] do the splitting and filtering during initialization
64cf306 [lianhuiwang] use defaultClassLoader to load classes of classesToRegister in KryoSerializer

core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala

@@ -58,14 +58,6 @@ class KryoSerializer(conf: SparkConf)
   private val classesToRegister = conf.get("spark.kryo.classesToRegister", "")
     .split(',')
     .filter(!_.isEmpty)
-    .map { className =>
-      try {
-        Class.forName(className)
-      } catch {
-        case e: Exception =>
-          throw new SparkException("Failed to load class to register with Kryo", e)
-      }
-    }
 
   def newKryoOutput() = new KryoOutput(bufferSize, math.max(bufferSize, maxBufferSize))
 
@@ -97,7 +89,8 @@ class KryoSerializer(conf: SparkConf)
       // Use the default classloader when calling the user registrator.
       Thread.currentThread.setContextClassLoader(classLoader)
       // Register classes given through spark.kryo.classesToRegister.
-      classesToRegister.foreach { clazz => kryo.register(clazz) }
+      classesToRegister
+        .foreach { className => kryo.register(Class.forName(className, true, classLoader)) }
       // Allow the user to register their own classes by setting spark.kryo.registrator.
       userRegistrator
         .map(Class.forName(_, true, classLoader).newInstance().asInstanceOf[KryoRegistrator])