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. 

throw Exception(ErrorCodes::SUPPORT_IS_DISABLED, "ClickHouse server built without NuRaft library. Cannot use internal coordination.");
#endif
    }
    for (auto & server : servers_to_start_before_tables)
    {
        server.start();
        LOG_INFO(log, "Listening for {}", server.getDescription());
    }
    
    auto & access_control = global_context->getAccessControl();
    try
    {
        access_control.setUpFromMainConfig(config(), config_path, [&] { return global_context->getZooKeeper(); });
    }
    catch (...)
    {
        tryLogCurrentException(log);
        throw;
    }
    
    global_context->setConfigReloadCallback([&]()
    {
        main_config_reloader->reload();
        access_control.reload();
    });
    
    global_context->getProcessList().setMaxSize(config().getInt("max_concurrent_queries", 0));
    
    
    double cache_size_to_ram_max_ratio = config().getDouble("cache_size_to_ram_max_ratio", 0.5);