plugin/forward: add it (#1447)

* plugin/forward: add it

This moves coredns/forward into CoreDNS. Fixes as a few bugs, adds a
policy option and more tests to the plugin.

Update the documentation, test IPv6 address and add persistent tests.

* Always use random policy when spraying

* include scrub fix here as well

* use correct var name

* Code review

* go vet

* Move logging to metrcs

* Small readme updates

* Fix readme

plugin/forward/persistent_test.go

@@ -0,0 +1,44 @@
+package forward
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+
+	"github.com/miekg/dns"
+)
+
+func TestPersistent(t *testing.T) {
+	s := dnstest.NewServer(func(w dns.ResponseWriter, r *dns.Msg) {
+		ret := new(dns.Msg)
+		ret.SetReply(r)
+		w.WriteMsg(ret)
+	})
+	defer s.Close()
+
+	h := newHost(s.Addr)
+	tr := newTransport(h)
+	defer tr.Stop()
+
+	c1, _ := tr.Dial("udp")
+	c2, _ := tr.Dial("udp")
+	c3, _ := tr.Dial("udp")
+
+	tr.Yield(c1)
+	tr.Yield(c2)
+	tr.Yield(c3)
+
+	if x := tr.Len(); x != 3 {
+		t.Errorf("Expected cache size to be 3, got %d", x)
+	}
+
+	tr.Dial("udp")
+	if x := tr.Len(); x != 2 {
+		t.Errorf("Expected cache size to be 2, got %d", x)
+	}
+
+	tr.Dial("udp")
+	if x := tr.Len(); x != 1 {
+		t.Errorf("Expected cache size to be 2, got %d", x)
+	}
+}