[SPARK-4999][Streaming] Change storeInBlockManager to false by default

Currently WAL-backed block is read out from HDFS and put into BlockManger with storage level MEMORY_ONLY_SER by default, since WAL-backed block is already materialized in HDFS with fault-tolerance, no need to put into BlockManger again by default.

Author: jerryshao <saisai.shao@intel.com>

Closes #3906 from jerryshao/SPARK-4999 and squashes the following commits:

b95f95e [jerryshao] Change storeInBlockManager to false by default

streaming/src/main/scala/org/apache/spark/streaming/dstream/ReceiverInputDStream.scala

@@ -86,7 +86,7 @@ abstract class ReceiverInputDStream[T: ClassTag](@transient ssc_ : StreamingCont
           }.toArray
           // Since storeInBlockManager = false, the storage level does not matter.
           new WriteAheadLogBackedBlockRDD[T](ssc.sparkContext,
-            blockIds, logSegments, storeInBlockManager = true, StorageLevel.MEMORY_ONLY_SER)
+            blockIds, logSegments, storeInBlockManager = false, StorageLevel.MEMORY_ONLY_SER)
         } else {
           new BlockRDD[T](ssc.sc, blockIds)
         }