make nydus blob size as small as possible

[liubogithub]

For the same dataset, the smaller nydus blob size is , the more data chunks can be downloaded in a single amplification. That is vital to the performance of ondemand read.

Now that we support zstd compression(#388 ), which in theory can reduce our blob size by about 10%, we're looking for other approaches to reduce nydus blob size further down.