Container construction fails with non-MySQLi drivers

Due to the fact that db implicitly depends on cache which is not available yet during container construction, it results in a fatal error:

( ! ) Fatal error: Call to a member function sql_exists() on a non-object in /var/www/qi/boards/z11/includes/db/mysql.php on line 252

MySQLi does not have this problem, because of an extra check that is mostly coincidental, which prevents the cache to be hit.

We should make cache available during container construction. The real fix is to inject the cache to the db properly or even decouple it completely and wrap a cached db driver decorator around the db connection.