[SPARK-5845][Shuffle] Time to cleanup spilled shuffle files not included in shuffle write time

I've added a timer in the right place to fix this inaccuracy.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #4965 from ilganeli/SPARK-5845 and squashes the following commits:

bfabf88 [Ilya Ganelin] Changed to using a foreach vs. getorelse
3e059b0 [Ilya Ganelin] Switched to using getorelse
b946d08 [Ilya Ganelin] Fixed error with option
9434b50 [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-5845
db8647e [Ilya Ganelin] Added update for shuffleWriteTime around spilled file cleanup in ExternalSorter

core/src/main/scala/org/apache/spark/shuffle/sort/SortShuffleWriter.scala

@@ -88,7 +88,10 @@ private[spark] class SortShuffleWriter[K, V, C](
     } finally {
       // Clean up our sorter, which may have its own intermediate files
       if (sorter != null) {
+        val startTime = System.nanoTime()
         sorter.stop()
+        context.taskMetrics.shuffleWriteMetrics.foreach(
+          _.incShuffleWriteTime(System.nanoTime - startTime))
         sorter = null
       }
     }