[SPARK-13279] Remove O(n^2) operation from scheduler.

This commit removes an unnecessary duplicate check in addPendingTask that meant
that scheduling a task set took time proportional to (# tasks)^2.

Author: Sital Kedia <skedia@fb.com>

Closes #11167 from sitalkedia/fix_stuck_driver and squashes the following commits:

3fe1af8 [Sital Kedia] [SPARK-13279] Remove unnecessary duplicate check in addPendingTask function

core/src/main/scala/org/apache/spark/scheduler/TaskSetManager.scala

@@ -114,9 +114,14 @@ private[spark] class TaskSetManager(
   // treated as stacks, in which new tasks are added to the end of the
   // ArrayBuffer and removed from the end. This makes it faster to detect
   // tasks that repeatedly fail because whenever a task failed, it is put
-  // back at the head of the stack. They are also only cleaned up lazily;
-  // when a task is launched, it remains in all the pending lists except
-  // the one that it was launched from, but gets removed from them later.
+  // back at the head of the stack. These collections may contain duplicates
+  // for two reasons:
+  // (1): Tasks are only removed lazily; when a task is launched, it remains
+  // in all the pending lists except the one that it was launched from.
+  // (2): Tasks may be re-added to these lists multiple times as a result
+  // of failures.
+  // Duplicates are handled in dequeueTaskFromList, which ensures that a
+  // task hasn't already started running before launching it.
   private val pendingTasksForExecutor = new HashMap[String, ArrayBuffer[Int]]
 
   // Set of pending tasks for each host. Similar to pendingTasksForExecutor,
@@ -181,9 +186,7 @@ private[spark] class TaskSetManager(
   private def addPendingTask(index: Int) {
     // Utility method that adds `index` to a list only if it's not already there
     def addTo(list: ArrayBuffer[Int]) {
-      if (!list.contains(index)) {
-        list += index
-      }
+      list += index
     }
 
     for (loc <- tasks(index).preferredLocations) {