Drainer: SyncDDL switcher does not work when syncing to oracle

[cartersz]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.
   set SyncDDL= false in configuration file.
   on TiDB client, you operation non "truncate table **" statement, the drainer will stop.
   See: fix sync-ddl control #1130

2. What did you expect to see?
   the drainer will not stop

3. What did you see instead?
   the drainer will stop

4. Please provide the relate downstream type and version of drainer.
   (run drainer -V in terminal to get drainer's version)