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.

middleware/secondary/setup.go

@@ -0,0 +1,82 @@
+package secondary
+
+import (
+	"github.com/miekg/coredns/core/dnsserver"
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/coredns/middleware/file"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("secondary", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	zones, err := secondaryParse(c)
+	if err != nil {
+		return err
+	}
+
+	// Add startup functions to retrieve the zone and keep it up to date.
+	for _, n := range zones.Names {
+		if len(zones.Z[n].TransferFrom) > 0 {
+			c.OnStartup(func() error {
+				zones.Z[n].StartupOnce.Do(func() {
+					zones.Z[n].TransferIn()
+					go func() {
+						zones.Z[n].Update()
+					}()
+				})
+				return nil
+			})
+		}
+	}
+
+	dnsserver.GetConfig(c).AddMiddleware(func(next dnsserver.Handler) dnsserver.Handler {
+		return Secondary{file.File{Next: next, Zones: zones}}
+	})
+
+	return nil
+}
+
+func secondaryParse(c *caddy.Controller) (file.Zones, error) {
+	z := make(map[string]*file.Zone)
+	names := []string{}
+	origins := []string{}
+	for c.Next() {
+		if c.Val() == "secondary" {
+			// secondary [origin]
+			origins = make([]string, len(c.ServerBlockKeys))
+			copy(origins, c.ServerBlockKeys)
+			args := c.RemainingArgs()
+			if len(args) > 0 {
+				origins = args
+			}
+			for i, _ := range origins {
+				origins[i] = middleware.Host(origins[i]).Normalize()
+				z[origins[i]] = file.NewZone(origins[i], "stdin")
+				names = append(names, origins[i])
+			}
+
+			for c.NextBlock() {
+				t, f, e := file.TransferParse(c)
+				if e != nil {
+					return file.Zones{}, e
+				}
+				for _, origin := range origins {
+					if t != nil {
+						z[origin].TransferTo = append(z[origin].TransferTo, t...)
+					}
+					if f != nil {
+						z[origin].TransferFrom = append(z[origin].TransferFrom, f...)
+					}
+				}
+			}
+		}
+	}
+	return file.Zones{Z: z, Names: names}, nil
+}