Make CoreDNS a server type plugin for Caddy (#220)

* Make CoreDNS a server type plugin for Caddy

Remove code we don't need and port all middleware over. Fix all tests
and rework the documentation.

Also make `go generate` build a caddy binary which we then copy into
our directory. This means `go build`-builds remain working as-is.

And new etc instances in each etcd test for better isolation.
Fix more tests and rework test.Server with the newer support Caddy offers.

Fix Makefile to support new mode of operation.

core/dnsserver/middleware.go

@@ -0,0 +1,52 @@
+package dnsserver
+
+import (
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+type (
+	// Middleware is the middle layer which represents the traditional
+	// idea of middleware: it chains one Handler to the next by being
+	// passed the next Handler in the chain.
+	Middleware func(Handler) Handler
+
+	// Handler is like dns.Handler except ServeDNS may return an rcode
+	// and/or error.
+	//
+	// If ServeDNS writes to the response body, it should return a status
+	// code. If the status code is not one of the following:
+	// * SERVFAIL (dns.RcodeServerFailure)
+	// * REFUSED (dns.RecodeRefused)
+	// * FORMERR (dns.RcodeFormatError)
+	// * NOTIMP (dns.RcodeNotImplemented)
+	//
+	// CoreDNS assumes *no* reply has yet been written. All other response
+	// codes signal other handlers above it that the response message is
+	// already written, and that they should not write to it also.
+	//
+	// If ServeDNS encounters an error, it should return the error value
+	// so it can be logged by designated error-handling middleware.
+	//
+	// If writing a response after calling another ServeDNS method, the
+	// returned rcode SHOULD be used when writing the response.
+	//
+	// If handling errors after calling another ServeDNS method, the
+	// returned error value SHOULD be logged or handled accordingly.
+	//
+	// Otherwise, return values should be propagated down the middleware
+	// chain by returning them unchanged.
+	Handler interface {
+		ServeDNS(context.Context, dns.ResponseWriter, *dns.Msg) (int, error)
+	}
+
+	// HandlerFunc is a convenience type like dns.HandlerFunc, except
+	// ServeDNS returns an rcode and an error. See Handler
+	// documentation for more information.
+	HandlerFunc func(context.Context, dns.ResponseWriter, *dns.Msg) (int, error)
+)
+
+// ServeDNS implements the Handler interface.
+func (f HandlerFunc) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	return f(ctx, w, r)
+}