plugin/rewrite: use request.Request and other cleanups (#1920)

This was done anyway, but only deep in the functions, just do this
everywhere; allows for shorter code and request.Request allows for
caching as well.

Cleanups, make it more Go like.
* remove unneeded switches
* remove testdir (why was this there??)
* simplify the logic
* remove unneeded variables
* put short functions on a single line
* fix documentation.
* spin off wire funcs in wire.go, make them functions.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/rewrite/rewrite.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
 )
@@ -38,8 +39,10 @@ type Rewrite struct {
 // ServeDNS implements the plugin.Handler interface.
 func (rw Rewrite) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
 	wr := NewResponseReverter(w, r)
+	state := request.Request{W: w, Req: r}
+
 	for _, rule := range rw.Rules {
-		switch result := rule.Rewrite(w, r); result {
+		switch result := rule.Rewrite(state); result {
 		case RewriteDone:
 			respRule := rule.GetResponseRule()
 			if respRule.Active == true {
@@ -68,7 +71,7 @@ func (rw Rewrite) Name() string { return "rewrite" }
 // Rule describes a rewrite rule.
 type Rule interface {
 	// Rewrite rewrites the current request.
-	Rewrite(dns.ResponseWriter, *dns.Msg) Result
+	Rewrite(state request.Request) Result
 	// Mode returns the processing mode stop or continue.
 	Mode() string
 	// GetResponseRule returns the rule to rewrite response with, if any.