[SPARK-4873][Streaming] Use `Future.zip` instead of `Future.flatMap`(for-loop)...

[SPARK-4873][Streaming] Use `Future.zip` instead of `Future.flatMap`(for-loop) in WriteAheadLogBasedBlockHandler

Use `Future.zip` instead of `Future.flatMap`(for-loop). `zip` implies these two Futures will run concurrently, while `flatMap` usually means one Future depends on the other one.

Author: zsxwing <zsxwing@gmail.com>

Closes #3721 from zsxwing/SPARK-4873 and squashes the following commits:

46a2cd9 [zsxwing] Use Future.zip instead of Future.flatMap(for-loop)

streaming/src/main/scala/org/apache/spark/streaming/receiver/ReceivedBlockHandler.scala

@@ -187,10 +187,7 @@ private[streaming] class WriteAheadLogBasedBlockHandler(
     }
 
     // Combine the futures, wait for both to complete, and return the write ahead log segment
-    val combinedFuture = for {
-      _ <- storeInBlockManagerFuture
-      fileSegment <- storeInWriteAheadLogFuture
-    } yield fileSegment
+    val combinedFuture = storeInBlockManagerFuture.zip(storeInWriteAheadLogFuture).map(_._2)
     val segment = Await.result(combinedFuture, blockStoreTimeout)
     WriteAheadLogBasedStoreResult(blockId, segment)
   }