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. 

{
    WriteLock lock{rwlock, lock_timeout};
    if (!lock)
        throw Exception("Lock timeout exceeded", ErrorCodes::TIMEOUT_EXCEEDED);
    
    loadIndices(lock);
    
    saveFileSizes(lock);
    try
    {
        fs::path data_path_in_backup_fs = data_path_in_backup;
        
        auto old_data_size = file_checker.getFileSize(data_file_path);
        {
            String file_path_in_backup = data_path_in_backup_fs / fileName(data_file_path);
            if (!backup->fileExists(file_path_in_backup))
                throw Exception(ErrorCodes::CANNOT_RESTORE_TABLE, "File {} in backup is required to restore table", file_path_in_backup);
            auto backup_entry = backup->readFile(file_path_in_backup);
            auto in = backup_entry->getReadBuffer();
            auto out = disk->writeFile(data_file_path, max_compress_block_size, WriteMode::Append);
            copyData(*in, *out);
        }
        
        {