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

middleware/reverse/setup.go

@@ -40,8 +40,7 @@ func reverseParse(c *caddy.Controller) (nets networks, fall bool, err error) {
 	zones := make([]string, len(c.ServerBlockKeys))
 
 	for i, str := range c.ServerBlockKeys {
-		host, _, _ := net.SplitHostPort(str)
-		zones[i] = strings.ToLower(host)
+		zones[i] = middleware.Host(str).Normalize()
 	}
 
 	for c.Next() {