cdc fail to observe pd leader migration

[lidezhu]

What did you do?

1. pd leader migrate from pd0 to pd1;
2. scale-in pd 0;

What did you expect to see?

cdc works normally;

What did you see instead?

cdc resign owner and cause some lag;

Versions of the cluster

Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

(paste TiDB cluster version here)

Upstream TiKV version (execute tikv-server --version):

(paste TiKV version here)

TiCDC version (execute cdc version):

(paste TiCDC version here)