Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

#pragma once
#include <Databases/DatabaseOnDisk.h>
#include <Common/ThreadPool.h>
namespace DB
{
/** Default engine of databases.
  * It stores tables list in filesystem using list of .sql files,
  *  that contain declaration of table represented by SQL ATTACH TABLE query.
  */
class DatabaseOrdinary : public DatabaseOnDisk