cdc: add index ddl lost when meeting error

[wk989898]

What did you do?

run sql "CREATE INDEX xxx"

What did you expect to see?

found the index on the downstream

What did you see instead?

The DDL lost.
Lots of Execute DDL error in the log

［mysql_ddl_sink.go:147］［"Execute DDL with error, retry later"]
 ....
［mysql_ddl_sink.go:91] ["async add index ddl is still running"]

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):

8.5.4

[wk989898]

#11476 introduced this bug, and #12123 should fix this bug.