SPARK-785 [CORE] ClosureCleaner not invoked on most PairRDDFunctions

This looked like perhaps a simple and important one. `combineByKey` looks like it should clean its arguments' closures, and that in turn covers apparently all remaining functions in `PairRDDFunctions` which delegate to it.

Author: Sean Owen <sowen@cloudera.com>

Closes #3690 from srowen/SPARK-785 and squashes the following commits:

8df68fe [Sean Owen] Clean context of most remaining functions in PairRDDFunctions, which ultimately call combineByKey

core/src/main/scala/org/apache/spark/rdd/PairRDDFunctions.scala

@@ -84,7 +84,10 @@ class PairRDDFunctions[K, V](self: RDD[(K, V)])
         throw new SparkException("Default partitioner cannot partition array keys.")
       }
     }
-    val aggregator = new Aggregator[K, V, C](createCombiner, mergeValue, mergeCombiners)
+    val aggregator = new Aggregator[K, V, C](
+      self.context.clean(createCombiner),
+      self.context.clean(mergeValue),
+      self.context.clean(mergeCombiners))
     if (self.partitioner == Some(partitioner)) {
       self.mapPartitions(iter => {
         val context = TaskContext.get()