Lightning does not use auto-sync and fails when the PD cluster address changed

[lichunzhu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Start a TiDB cluster with 3 PDs ① ② ③ and a Lightning connected
2. Scale-out 3 more PDs ④ ⑤ ⑥
3. Wait 31 seconds
4. Scale-in the original PDs ① ② ③
5. Wait 31 seconds
6. Keep Lightning running.

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

We make sure lightning succeed.

3. What did you see instead (Required)

Lightning will fail in some cases due to no valid pd-addr.

4. What is your TiDB version? (Required)

master