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/protocol.go

@@ -0,0 +1,30 @@
+package forward
+
+// Copied from coredns/core/dnsserver/address.go
+
+import (
+	"strings"
+)
+
+// protocol returns the protocol of the string s. The second string returns s
+// with the prefix chopped off.
+func protocol(s string) (int, string) {
+	switch {
+	case strings.HasPrefix(s, _tls+"://"):
+		return TLS, s[len(_tls)+3:]
+	case strings.HasPrefix(s, _dns+"://"):
+		return DNS, s[len(_dns)+3:]
+	}
+	return DNS, s
+}
+
+// Supported protocols.
+const (
+	DNS = iota + 1
+	TLS
+)
+
+const (
+	_dns = "dns"
+	_tls = "tls"
+)