There is cross-location traffic when workload starts

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Create a cluster with 3 TiDB and 3 TiProxy
2. Set zone labels for TiDB and TiProxy
3. Start a workload, like sysbench with 100 threads

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

There is no cross-location traffic all the time

3. What did you see instead (Required)

There is cross-location traffic at the beginning.

4. What is your version? (Required)

master