is it suitable to use specified compression for snapshot SST implicitly?

[siddontang]

Now we forcibly use fast compression for snapshot SST, but some users may don't want use any compression. We may use the same compression set in the config file.

/cc @zhangjinpeng1987 @huachaohuang

[zhangjinpeng87]

We used a fast compression for snapshot SST was partly because we don't want to use an expensive compression which will take some time to generate the snapshot, and also we don't want the content is too large when delivering on network.

[siddontang]

If the user doesn't install LZ4 or want to use their own compression type later, how can we support?

[siddontang]

Seem it is hard for the user to add their own compression lib, they should not only change rust-rocksdb, but also need to change TiKV in some places.

See tikv/rust-rocksdb#152

[huachaohuang]

We actually choose the fast compression algorithm from supported algorithms, which sounds reasonable to me. However, in that specific case, I guess something may be wrong, but I am not sure what's wrong.