Changed the type of the persistentRdds hashmap back to

TimeStampedHashMap.

core/src/main/scala/spark/SparkContext.scala

@@ -100,7 +100,7 @@ class SparkContext(
   private[spark] val addedJars = HashMap[String, Long]()
 
   // Keeps track of all persisted RDDs
-  private[spark] val persistentRdds: ConcurrentMap[Int, RDD[_]] = new ConcurrentHashMap[Int, RDD[_]]
+  private[spark] val persistentRdds = new TimeStampedHashMap[Int, RDD[_]]
   private[spark] val metadataCleaner = new MetadataCleaner("SparkContext", this.cleanup)