add foreign key need multi-schema change

[crazycs520]

Dev Task

At the same point in time, there may be two versions of the schema in the TiDB cluster, so we can't create foreign key in one schema version, since this may break foreign key constraint, such as delete reference table
without foreign key constrain check in child table.

-- In TiDB-1 and Schema Version is 1
insert into t_has_foreign_key values (1, 1);
-- In TiDB-0 and  Schema Version is 0
delete from t_reference where id = 1; --Since doesn't know foreign key information in old version, so doesn't do foreign key constrain check.