Constrained Application Protocol Client and Server for go

You can read more about CoAP in RFC 7252. I also did some preliminary work on SUBSCRIBE support from an early draft.

Benchmark

goos: darwin
goarch: amd64
BenchmarkQps
BenchmarkQps-8   	   96454	     12558 ns/op	     360 B/op	       9 allocs/op
PASS