Support Zstd newer than 1.4.4

[Arkiver2]

Supporting a version of Zstd requires some testing to make sure all works well.

[sfan5]

Out of curiosity, what kind of problems do you expect?
Every Zstd version is used on probably millions of devices so I'd think that updating e.g. within a minor version (to 1.4.9) is close to 100% safe.

[TheTechRobo]

@sfan5 I think one issue is compression ratios - I heard that they were worried about worse compression ratios. I'm not sure if that's the issue here or not.