Experiment with alternative compression

[kermitt2]

We use snappy right now for LMDB stored records. There are other compression methods that might be relevant to small objects to get higher compression ratio and faster decompression (in principle we are less interested in compression speed).

See for instance https://morotti.github.io/lzbench-web as a benchmark study. zstd (which has a training mode) and lz4 might be more interesting than snappy for our usage.