Add support for zstd

[phoenix-bjoern]

For compression multiple compression libraries are supported. For the Cm_Cache_Backend_Redis the zstd library has been added in 2018 (colinmollenhour/Cm_Cache_Backend_Redis@a9c4a5a) which IMHO gives a slightly better compression at a better speed.
It would be beneficial to add zstd for the Redis session module as well.

[colinmollenhour]

Sure, a PR for this would be quite welcome. Thanks