google cloud storage can't use parallel write

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

google cloud storage can't use parallel write, so the performance is poor. It's about 20~40MB/s.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master