db:schema:load is not idempotent

[Alex Kichkailo]

If you try to run db:schema:load multiple times, an error is thrown

DB::Exception: Table <DB>.ar_internal_metadata already exists. **(TABLE_ALREADY_EXISTS)**

It appears that the clickhouse_structure.sql file generated by the clickhouse-activerecord adapter is missing DROP TABLE IF EXISTS statements.

In Rails, db:schema:load is supposed to be idempotent so it could be used to reset the DB in a test or dev environment.