core: add more transports (#574)

* core: add listening for other protocols

Allow CoreDNS to listen for TLS request coming over port 853. This can
be enabled with `tls://` in the config file.

Implement listening for grps:// as well.

a Corefile like:

~~~
. tls://.:1853 {
    whoami
    tls
}
~~~

Means we listen on 1853 for tls requests, the `tls` config item allows
configuration for TLS parameters. We *might* be tempted to use Caddy's
Let's Encrypt implementation here.

* Refactor coredns/grpc into CoreDNS

This makes gRPC a first class citizen in CoreDNS. Add defines as being
just another server.

* some cleanups

* unexport the servers

* Move protobuf dir

* Hook up TLS properly

* Fix test

* listen for TLS as well. README updates

* disable test, fix package

* fix test

* Fix tests

* Fix remaining test

* Some tests

* Make the test work

* Add grpc test from #580

* fix crash

* Fix tests

* Close conn

* README cleanups

* README

* link RFC

core/dnsserver/server.go

@@ -61,7 +61,6 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 			stack = site.Middleware[i](stack)
 		}
 		site.middlewareChain = stack
-		site.Server = s
 	}
 
 	return s, nil
@@ -95,7 +94,7 @@ func (s *Server) ServePacket(p net.PacketConn) error {
 
 // Listen implements caddy.TCPServer interface.
 func (s *Server) Listen() (net.Listener, error) {
-	l, err := net.Listen("tcp", s.Addr)
+	l, err := net.Listen("tcp", s.Addr[len(TransportDNS+"://"):])
 	if err != nil {
 		return nil, err
 	}
@@ -104,7 +103,7 @@ func (s *Server) Listen() (net.Listener, error) {
 
 // ListenPacket implements caddy.UDPServer interface.
 func (s *Server) ListenPacket() (net.PacketConn, error) {
-	p, err := net.ListenPacket("udp", s.Addr)
+	p, err := net.ListenPacket("udp", s.Addr[len(TransportDNS+"://"):])
 	if err != nil {
 		return nil, err
 	}