block-allow-list doesn't skip blocked tables from online DDL

[lance6716]

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.

If user starts a DM task with online-ddl-scheme config, and the starting position can't fully see online DDLs of a tables. Though the table maybe blocked by blokc-allow-lists, DM will raise an error

see #1775

2. What did you expect to see?

no error because the table is blocked

3. What did you see instead?

"Message": "startLocation: [position: (mysql-bin.000001, 2257), gtid-set: 09bec856-ba95-11ea-850a-58f2b4af5188:1-8], endLocation: [position: (mysql-bin.000001, 2435), gtid-set: 09bec856-ba95-11ea-850a-58f2b4af5188:1-9], origin SQL: [rename table online_ddl.ignore to online_ddl._ignore_del, online_ddl._ignore_gho to online_ddl.ignore]: online ddl metadata for ghost temporary table online_ddl._ignore_gho not found"