zstd: explicitly disable checksum checking

[WojciechMula]

This is follow up of #556. In short: we're decompressing the same files over and over. We measured that checksumming takes ~15% of decompression time. We want to conditionally perform checksumming (likely: for the first time).

Would it be possible to add an option to the decoder, like WithChecksum(false)?

[klauspost]

Yes, sounds reasonable. I would rather have it be IgnoreChecksum(true), but that is a detail.

[WojciechMula]

Great! I will prepare a PR for this.