Check if reusing the encoder is worth it

[rklaehn]

A lot of effort has been spent reusing decoders. But encoders are always created from scratch. Check if reusing encoders is worth it, or if the benefit is marginal.

[rklaehn]

My hunch is that compression is such an expensive operation that avoiding the allocation of a few buffers does not make a big difference, unlike decompression with is very fast with zstd. If there is a difference, you are most likely to see it with low zstd_level values.