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 <Interpreters/Context_fwd.h>
#include <Databases/TablesLoader.h>
namespace DB
{
void loadMetadataSystem(ContextMutablePtr context);
void loadMetadata(ContextMutablePtr context, const String & default_database_name = {});
void startupSystemTables();
void maybeConvertOrdinaryDatabaseToAtomic(ContextMutablePtr context, const DatabasePtr & database);
}