Disable MySQL foreign key checks before dropping all tables

Fixes #20237. Disables MySQL foreign key checks before dropping all tables in a restore. MySQL doesn't honor `CASCADE` without a special flag when the database/tables are created. In order to drop the tables we need to disable foreign key checks. After the drop, re-enable the key checks.

WIP: Pending confirmation from customer that this fix works.

See merge request !5472
Signed-off-by: Rémy Coutable <remy@rymai.me>