[SPARK-8161] Set externalBlockStoreInitialized to be true, after ExternalBlockStore is initialized

externalBlockStoreInitialized is never set to be true, which causes the blocks stored in ExternalBlockStore can not be removed.

Author: Mingfei <mingfei.shi@intel.com>

Closes #6702 from shimingfei/SetTrue and squashes the following commits:

add61d8 [Mingfei] Set externalBlockStoreInitialized to be true, after ExternalBlockStore is initialized

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

@@ -83,8 +83,10 @@ private[spark] class BlockManager(
   private var externalBlockStoreInitialized = false
   private[spark] val memoryStore = new MemoryStore(this, maxMemory)
   private[spark] val diskStore = new DiskStore(this, diskBlockManager)
-  private[spark] lazy val externalBlockStore: ExternalBlockStore =
+  private[spark] lazy val externalBlockStore: ExternalBlockStore = {
+    externalBlockStoreInitialized = true
     new ExternalBlockStore(this, executorId)
+  }
 
   private[spark]
   val externalShuffleServiceEnabled = conf.getBoolean("spark.shuffle.service.enabled", false)