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 <Core/UUID.h>
#include <Databases/LoadingStrictnessLevel.h>
#include <Interpreters/Context_fwd.h>
#include <Parsers/IAST_fwd.h>
#include <Storages/IStorage_fwd.h>
#include <base/types.h>
#include <Common/Exception.h>
#include <Common/ThreadPool.h>
#include <ctime>
#include <functional>
#include <memory>
#include <mutex>
#include <vector>
#include <map>
namespace DB
{
struct Settings;
struct ConstraintsDescription;
struct IndicesDescription;
struct StorageInMemoryMetadata;
struct StorageID;
class ASTCreateQuery;
class AlterCommands;