[Hot Fix #42] Persisted RDD disappears on storage page if re-used

If a previously persisted RDD is re-used, its information disappears from the Storage page.

This is because the tasks associated with re-using the RDD do not report the RDD's blocks as updated (which is correct). On stage submit, however, we overwrite any existing information regarding that RDD with a fresh one, whether or not the information for the RDD already exists.

Author: Andrew Or <andrewor14@gmail.com>

Closes #281 from andrewor14/ui-storage-fix and squashes the following commits:

408585a [Andrew Or] Fix storage UI bug

core/src/main/scala/org/apache/spark/ui/storage/BlockManagerUI.scala

@@ -84,7 +84,7 @@ private[ui] class BlockManagerListener(storageStatusListener: StorageStatusListe
 
   override def onStageSubmitted(stageSubmitted: SparkListenerStageSubmitted) = synchronized {
     val rddInfo = stageSubmitted.stageInfo.rddInfo
-    _rddInfoMap(rddInfo.id) = rddInfo
+    _rddInfoMap.getOrElseUpdate(rddInfo.id, rddInfo)
   }
 
   override def onStageCompleted(stageCompleted: SparkListenerStageCompleted) = synchronized {