DM-worker should not retry on long time ddls directly

[lichunzhu]

What did you do?

Use DM-worker to replicate some long time costing ddls to downstream TiDB. For example, adding index ddls.

What did you expect to see?

DM-worker can replicate ddls successfully.

What did you see instead?

DM-worker fails because of i/o timeout and keeps retrying to replicate these ddls again.

Versions of the cluster

master

current status of DM cluster (execute query-status <task-name> in dmctl)

(paste current status of DM cluster here)

[lichunzhu]

Root Cause

When DM executes some time-costing ddls, especially for adding index, dm-worker may fail to get result because it won't return any result until it succeeds. If this time cost is larger than syncer's read-timeout, dm-worker will fail and try to execute ddl again. This will make the situation even worse.

Workaround

For these long time costing ddls, we can use a special connection to execute them and keep watching the result asynchronously through admin show ddl jobs.

[lance6716]

duplicated with #4689