[SPARK-2144] ExecutorsPage reports incorrect # of RDD blocks

This is reproducible whenever we drop a block because of memory pressure.

This is because StorageStatusListener actually never removes anything from the block maps of its StorageStatuses. Instead, when a block is dropped, it sets the block's storage level to `StorageLevel.NONE`, when it should just remove it from the map.

This PR includes this simple fix.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1080 from andrewor14/ui-blocks and squashes the following commits:

fcf9f1a [Andrew Or] Remove BlockStatus if it is no longer cached

core/src/main/scala/org/apache/spark/storage/StorageStatusListener.scala

@@ -37,7 +37,11 @@ class StorageStatusListener extends SparkListener {
     val filteredStatus = storageStatusList.find(_.blockManagerId.executorId == execId)
     filteredStatus.foreach { storageStatus =>
       updatedBlocks.foreach { case (blockId, updatedStatus) =>
-        storageStatus.blocks(blockId) = updatedStatus
+        if (updatedStatus.storageLevel == StorageLevel.NONE) {
+          storageStatus.blocks.remove(blockId)
+        } else {
+          storageStatus.blocks(blockId) = updatedStatus
+        }
       }
     }
   }