[ticket/11037] Eliminate global $db usage in cache drivers.

The only time $db is needed in cache drivers is to navigate the
result set in sql_save. Pass it as a parameter in that function.

PHPBB3-11037

phpBB/includes/cache/driver/null.php

@@ -105,9 +105,9 @@ class phpbb_cache_driver_null extends phpbb_cache_driver_base
 	}
 
 	/**
-	* Save sql query
+	* {@inheritDoc}
 	*/
-	function sql_save($query, $query_result, $ttl)
+	function sql_save(phpbb_db_driver $db, $query, $query_result, $ttl)
 	{
 		return $query_result;
 	}