[Minor][Core] fix comments in MapOutputTracker

Using driver and executor in the comments of ```MapOutputTracker``` is more clear.

Author: wangfei <wangfei1@huawei.com>

Closes #3700 from scwf/commentFix and squashes the following commits:

aa68524 [wangfei] master and worker should be driver and executor

core/src/main/scala/org/apache/spark/MapOutputTracker.scala

@@ -72,7 +72,7 @@ private[spark] class MapOutputTrackerMasterActor(tracker: MapOutputTrackerMaster
 /**
  * Class that keeps track of the location of the map output of
  * a stage. This is abstract because different versions of MapOutputTracker
- * (driver and worker) use different HashMap to store its metadata.
+ * (driver and executor) use different HashMap to store its metadata.
  */
 private[spark] abstract class MapOutputTracker(conf: SparkConf) extends Logging {
   private val timeout = AkkaUtils.askTimeout(conf)
@@ -81,11 +81,11 @@ private[spark] abstract class MapOutputTracker(conf: SparkConf) extends Logging
   var trackerActor: ActorRef = _
 
   /**
-   * This HashMap has different behavior for the master and the workers.
+   * This HashMap has different behavior for the driver and the executors.
    *
-   * On the master, it serves as the source of map outputs recorded from ShuffleMapTasks.
-   * On the workers, it simply serves as a cache, in which a miss triggers a fetch from the
-   * master's corresponding HashMap.
+   * On the driver, it serves as the source of map outputs recorded from ShuffleMapTasks.
+   * On the executors, it simply serves as a cache, in which a miss triggers a fetch from the
+   * driver's corresponding HashMap.
    *
    * Note: because mapStatuses is accessed concurrently, subclasses should make sure it's a
    * thread-safe map.
@@ -99,7 +99,7 @@ private[spark] abstract class MapOutputTracker(conf: SparkConf) extends Logging
   protected var epoch: Long = 0
   protected val epochLock = new AnyRef
 
-  /** Remembers which map output locations are currently being fetched on a worker. */
+  /** Remembers which map output locations are currently being fetched on an executor. */
   private val fetching = new HashSet[Int]
 
   /**
@@ -196,8 +196,8 @@ private[spark] abstract class MapOutputTracker(conf: SparkConf) extends Logging
 
   /**
    * Called from executors to update the epoch number, potentially clearing old outputs
-   * because of a fetch failure. Each worker task calls this with the latest epoch
-   * number on the master at the time it was created.
+   * because of a fetch failure. Each executor task calls this with the latest epoch
+   * number on the driver at the time it was created.
    */
   def updateEpoch(newEpoch: Long) {
     epochLock.synchronized {
@@ -229,7 +229,7 @@ private[spark] class MapOutputTrackerMaster(conf: SparkConf)
   private var cacheEpoch = epoch
 
   /**
-   * Timestamp based HashMap for storing mapStatuses and cached serialized statuses in the master,
+   * Timestamp based HashMap for storing mapStatuses and cached serialized statuses in the driver,
    * so that statuses are dropped only by explicit de-registering or by TTL-based cleaning (if set).
    * Other than these two scenarios, nothing should be dropped from this HashMap.
    */
@@ -339,7 +339,7 @@ private[spark] class MapOutputTrackerMaster(conf: SparkConf)
 }
 
 /**
- * MapOutputTracker for the workers, which fetches map output information from the driver's
+ * MapOutputTracker for the executors, which fetches map output information from the driver's
  * MapOutputTrackerMaster.
  */
 private[spark] class MapOutputTrackerWorker(conf: SparkConf) extends MapOutputTracker(conf) {