Add finalizers to prevent memory leaking

[ains-stripe]

We've seen some processes leaking memory when using this library.
After looking at this issue
klauspost/compress#264

It seems like it was fixed in kafka-go by using finalizers to ensure that Close was called in the wrapped objects when the GC cleans them up, otherwise the goroutines in compress will leak.
https://github.com/segmentio/kafka-go/blob/master/zstd/zstd.go#L39-L47

[mostynb]

Thanks for the fix! I will watch the referenced issue to see if this can be simplified later.

I wonder if we should be using WithDecoderConcurrency/WithEncoderConcurrency when creating these?

[ains-stripe]

That's another way of getting around the bug (disabling the concurrency) if you use any at all though you'll need to cleanup the goroutines.

Thanks for merging though! Would you mind tagging a new release?

[mostynb]

Would you mind tagging a new release?

Done.