Merge branch 'mysql_drop_all_tables' into 'master'

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