[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)

(cherry picked from commit b4d0db80a0bfba7f1e045d4edb9357b4b2c0a557)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>