[SPARK-5219][Core] Add locks to avoid scheduling race conditions

Author: zsxwing <zsxwing@gmail.com>

Closes #4019 from zsxwing/SPARK-5219 and squashes the following commits:

36a8b4e [zsxwing] Add locks to avoid race conditions

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

@@ -361,7 +361,7 @@ private[spark] class TaskSchedulerImpl(
     dagScheduler.executorHeartbeatReceived(execId, metricsWithStageIds, blockManagerId)
   }
 
-  def handleTaskGettingResult(taskSetManager: TaskSetManager, tid: Long) {
+  def handleTaskGettingResult(taskSetManager: TaskSetManager, tid: Long): Unit = synchronized {
     taskSetManager.handleTaskGettingResult(tid)
   }
 

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

@@ -542,7 +542,7 @@ private[spark] class TaskSetManager(
   /**
    * Check whether has enough quota to fetch the result with `size` bytes
    */
-  def canFetchMoreResults(size: Long): Boolean = synchronized {
+  def canFetchMoreResults(size: Long): Boolean = sched.synchronized {
     totalResultSize += size
     calculatedTasks += 1
     if (maxResultSize > 0 && totalResultSize > maxResultSize) {
@@ -671,7 +671,7 @@ private[spark] class TaskSetManager(
     maybeFinishTaskSet()
   }
 
-  def abort(message: String) {
+  def abort(message: String): Unit = sched.synchronized {
     // TODO: Kill running tasks if we were not terminated due to a Mesos error
     sched.dagScheduler.taskSetFailed(taskSet, message)
     isZombie = true