checkpoint lag increases to 7min when one upstream TiKV node fails

[hicqu]

What did you do?

Run an upstream cluster with 4 TiKV instances. There are about 85K region leaders on every instance.
Then creates a changefeed to subscribe those regions.
After a while, stop one TiKV node.

What did you expect to see?

TiCDC checkpoint lag can increase, but should become normal quickly.

What did you see instead?

TiCDC checkpoint lag increases to about 7min.

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)