core: remove dns.ServeMux (#576)
core: remove dns.ServeMux (#576)

We don't need to use the muxer in Go DNS s we only have one entry point
per server - and mux ourselves in ServeDNS. Also make ServeDNS take a
context, and make that the canonical way to call all middleware.
jremond/coredns,chrisohaver/coredns,ii/coredns,jremond/coredns,ii/coredns,yongtang/coredns,miekg/coredns,johnbelamaric/coredns,johnbelamaric/coredns,miekg/coredns,coredns/coredns,varyoo/coredns