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.

test/server.go

@@ -0,0 +1,91 @@
+package test
+
+import (
+	"net"
+	"sync"
+	"testing"
+	"time"
+
+	_ "github.com/miekg/coredns/core"
+
+	"github.com/mholt/caddy"
+	"github.com/miekg/dns"
+)
+
+func TCPServer(t *testing.T, laddr string) (*dns.Server, string, error) {
+	l, err := net.Listen("tcp", laddr)
+	if err != nil {
+		return nil, "", err
+	}
+
+	server := &dns.Server{Listener: l, ReadTimeout: time.Hour, WriteTimeout: time.Hour}
+
+	waitLock := sync.Mutex{}
+	waitLock.Lock()
+	server.NotifyStartedFunc = func() { t.Logf("started TCP server on %s", l.Addr()); waitLock.Unlock() }
+
+	go func() {
+		server.ActivateAndServe()
+		l.Close()
+	}()
+
+	waitLock.Lock()
+	return server, l.Addr().String(), nil
+}
+
+func UDPServer(t *testing.T, laddr string) (*dns.Server, string, error) {
+	pc, err := net.ListenPacket("udp", laddr)
+	if err != nil {
+		return nil, "", err
+	}
+	server := &dns.Server{PacketConn: pc, ReadTimeout: time.Hour, WriteTimeout: time.Hour}
+
+	waitLock := sync.Mutex{}
+	waitLock.Lock()
+	server.NotifyStartedFunc = func() { t.Logf("started UDP server on %s", pc.LocalAddr()); waitLock.Unlock() }
+
+	go func() {
+		server.ActivateAndServe()
+		pc.Close()
+	}()
+
+	waitLock.Lock()
+	return server, pc.LocalAddr().String(), nil
+}
+
+// CoreDNSServer returns a test server. It just takes a normal Corefile as input.
+func CoreDNSServer(corefile string) (*caddy.Instance, error) { return caddy.Start(NewInput(corefile)) }
+
+// CoreDNSSserverStop stops a server.
+func CoreDNSServerStop(i *caddy.Instance) { i.Stop() }
+
+// CoreDNSServeRPorts returns the ports the instance is listening on. The integer k indicates
+// which ServerListener you want.
+func CoreDNSServerPorts(i *caddy.Instance, k int) (udp, tcp string) {
+	srvs := i.Servers()
+	if len(srvs) < k+1 {
+		return "", ""
+	}
+	u := srvs[k].LocalAddr()
+	t := srvs[k].Addr()
+
+	if u != nil {
+		udp = u.String()
+	}
+	if t != nil {
+		tcp = t.String()
+	}
+	return
+}
+
+type Input struct {
+	corefile []byte
+}
+
+func NewInput(corefile string) *Input {
+	return &Input{corefile: []byte(corefile)}
+}
+
+func (i *Input) Body() []byte       { return i.corefile }
+func (i *Input) Path() string       { return "Corefile" }
+func (i *Input) ServerType() string { return "dns" }