snap_br: snapshot backup may fail if TiKV is temporary unreachable

[YuJuncen]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Make a temporary network isolation between BR and PD.
2. Start snapshot backup.

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

It should success after the network isolation resumed.

3. What did you see instead (Required)

BR failed.

4. What is your TiDB version? (Required)

Master