flashback does not properly handle dropped tables

[Defined2014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

use test;
create table t(a int);
select now();
insert into t values (1);
drop table t;
flashback cluster to timestamp 'xxx'; // flashback to timestamp before insert
select * from t;

2. What did you expect to see? (Required)

mysql> select * from t;
Empty set (0.01 sec)

3. What did you see instead (Required)

mysql> select * from t;
+------+
| a    |
+------+
|    1 |
+------+
1 row in set (0.01 sec)

4. What is your TiDB version? (Required)

6ae151a

[Defined2014]

Also find a bug with flashback dropped schema. I need to rethink the flashback key ranges.