* ddl: add different state tests
* ddl: update
* ddl: clean up
* ddl: add a comment
* ddl: make more readable
* ddl: add a comment
* ddl: add the error check
* ddl: address comments
DDLs that doesn't modify rows like create/drop/truncate table, create/drop database
do not need to wait 2 lease on ddl worker, we can safely remove those wait to
speed up DDL.
But user may create a table, then insert data on that table on another TiDB server,
So we need to wait in session.
User can set a session variable to skip DDL wait if needed, and there is no risk of
data inconsistency.
This solution remove the need for user to set lease to zero when loading data,
It's much more safe and easy to use.
