nsqd: Use Klaus Post's compression libraries

We would quite like to use compression with NSQ to save on data transfer costs, but the CPU impact is higher than we'd like. Our experiments have shown that Klaus Post's compression libraries perform much better than the standard library Deflate and Google's Snappy, with the sweet spot appearing to be level 3 flate compressing our traffic to about 25% of its original size, but only incurring a CPU cost equivalent to Snappy.

Would there be any interest in taking a PR that makes this change?