zstd: pooledZipWriter should return Writers to the same pool

[mostynb]

This code was previously Get'ing objects from one pool and later Put'ing them to a different pool, so they were never reused.

[mostynb]

Also, pooledZipReader seems to be unused because it leaks goroutines- would you be interested in merging something like https://github.com/mostynb/zstdpool-syncpool/blob/main/decoderpool.go (as an un-exported type)?

[klauspost]

@mostynb Nice! Thanks!

I don't want to rely on finalizers. I want to fix it properly, ie have a streaming option that only uses the main goroutine.

[mostynb]

I don't want to rely on finalizers. I want to fix it properly, ie have a streaming option that only uses the main goroutine.

I agree that would be preferable. Is it worth commenting out pooledZipReader in the meantime, since it's unused?

[klauspost]

@mostynb It is fine to leave. Thanks for the contribution! I will include it in the release I will do shortly.