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. 

plain(disk->writeFile(data_path, max_compress_block_size, WriteMode::Append)),
            compressed(*plain, std::move(codec), max_compress_block_size),
            plain_offset(initial_data_size)
        {
        }
        std::unique_ptr<WriteBuffer> plain;
        CompressedWriteBuffer compressed;
        
        size_t plain_offset;
        
        bool written = false;
        void finalize()