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/host.go

@@ -0,0 +1,44 @@
+package forward
+
+import (
+	"crypto/tls"
+	"sync"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+type host struct {
+	addr   string
+	client *dns.Client
+
+	tlsConfig *tls.Config
+	expire    time.Duration
+
+	fails uint32
+	sync.RWMutex
+	checking bool
+}
+
+// newHost returns a new host, the fails are set to 1, i.e.
+// the first healthcheck must succeed before we use this host.
+func newHost(addr string) *host {
+	return &host{addr: addr, fails: 1, expire: defaultExpire}
+}
+
+// setClient sets and configures the dns.Client in host.
+func (h *host) SetClient() {
+	c := new(dns.Client)
+	c.Net = "udp"
+	c.ReadTimeout = 2 * time.Second
+	c.WriteTimeout = 2 * time.Second
+
+	if h.tlsConfig != nil {
+		c.Net = "tcp-tls"
+		c.TLSConfig = h.tlsConfig
+	}
+
+	h.client = c
+}
+
+const defaultExpire = 10 * time.Second