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. 

{
    if (indices_loaded)
        return;
    if (disk->exists(index_file_path))
    {
        CompressedReadBufferFromFile index_in(disk->readFile(index_file_path, ReadSettings{}.adjustBufferSize(4096)));
        indices.read(index_in);
    }
    indices_loaded = true;
    num_indices_saved = indices.blocks.size();
    
    updateTotalRows(lock);
}
void StorageStripeLog::saveIndices(const WriteLock & )
{
    size_t num_indices = indices.blocks.size();
    if (num_indices_saved == num_indices)
        return;
    size_t start = num_indices_saved;
    auto index_out_compressed = disk->writeFile(index_file_path, DBMS_DEFAULT_BUFFER_SIZE, WriteMode::Append);
    auto index_out = std::make_unique<CompressedWriteBuffer>(*index_out_compressed);
    for (size_t i = start; i != num_indices; ++i)
        indices.blocks[i].write(*index_out);
    index_out->next();
    index_out_compressed->next();
    index_out_compressed->finalize();
    num_indices_saved = num_indices;
}
void StorageStripeLog::removeUnsavedIndices(const WriteLock & )