Unnecessary rollback and Fail to drop table from schema tracker when pause task

[GMHDBJD]

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.

insert into tb1 values (1)
insert into tb2 values (2)
stop-task
start-task
insert into tb1 values(3)
pause-task

2. What did you expect to see?
   no need to rollback, no error log
3. What did you see instead?
   table 2 failed to rollback

[2021/08/24 23:01:30.377 -04:00] [DEBUG] [checkpoint.go:691] ["try to rollback checkpoint"] [task=test] [unit="binlog replication"] [component="remote checkpoint"] [schema=shardddl1] [table=tb1] [checkpoint="position: (mysql-bin.000001, 1990), gtid-set: (flushed position: (mysql-bin.000001, 1990), gtid-set: )"]
[2021/08/24 23:01:30.378 -04:00] [DEBUG] [checkpoint.go:691] ["try to rollback checkpoint"] [task=test] [unit="binlog replication"] [component="remote checkpoint"] [schema=shardddl1] [table=tb2] [checkpoint="position: (mysql-bin.000001, 1735), gtid-set: (flushed position: (mysql-bin.000001, 1735), gtid-set: )"]
[2021/08/24 23:01:30.378 -04:00] [INFO] [checkpoint.go:694] ["rollback checkpoint"] [task=test] [unit="binlog replication"] [component="remote checkpoint"] [schema=shardddl1] [table=tb2] [from="position: (mysql-bin.000001, 1735), gtid-set: "] [to="position: (mysql-bin.000001, 1735), gtid-set: "]
[2021/08/24 23:01:30.378 -04:00] [WARN] [checkpoint.go:697] ["failed to drop table from schema tracker"] [task=test] [unit="binlog replication"] [component="remote checkpoint"] [schema=shardddl1] [table=tb2] [error="[schema:1146]Table 'shardddl1.tb2' doesn't exist"]

4. Root Cause.
   We use lazy init checkpoint schema now, so the case is that the table info exists in table checkpoint but not exists in schema tracker. In this situation, we don't need to rollback it. Meanwhile, we now no need to create the table in schema tracker during rollback, just simply delete the table and create after task resume.

[lance6716]

in https://asktug.com/t/topic/182909 , DM v2.0.1 drops 2600 tables costing 1h25m 😂 (hope in later version, each DDL will not wait 2 seconds to finish)

[lance6716]

And if the rolling back costs a lots of time, the progress will be displayed "Running" because Syncer.Process is not exited, which is misleading to user.