zstd.go initializes shared global encoder+decoder regardless of whether zstd is being used

[dnwe]

Currently we initialise a global zstd encoder+decoder at import time:
https://github.com/Shopify/sarama/blob/8dbbfb50f4698c577c49ff8142c90a9185eb3114/zstd.go#L7-L8

Under the covers the compress library spins up goroutines to handle the decompression/compression

If the Sarama user doesn't ever intend to use zstd then this is a bunch of wasted resource.

I'm also not sure if a shared global encoder+decoder is the correct behaviour, or whether we should have per-producer encoders and WithEncoderConcurrency(1) and per-consumer decoders and WithDecoderConcurrency(1)

I also wonder if we should be surfacing the WithDecoderLowmem and WithWindowSize options in sarama.Config{...} so that users can tune the performance vs memory usage tradeoffs themselves.

[github-actions]

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