p2p: msg.Payload contains list data

With RLPx frames, the message code is contained in the
frame and is no longer part of the encoded data.

EncodeMsg, Msg.Decode have been updated to match.
Code that decodes RLP directly from Msg.Payload will need
to change.

p2p/message.go

@@ -51,19 +51,8 @@ type Msg struct {
 
 // NewMsg creates an RLP-encoded message with the given code.
 func NewMsg(code uint64, params ...interface{}) Msg {
-	buf := new(bytes.Buffer)
-	for _, p := range params {
-		buf.Write(ethutil.Encode(p))
-	}
-	return Msg{Code: code, Size: uint32(buf.Len()), Payload: buf}
-}
-
-func encodePayload(params ...interface{}) []byte {
-	buf := new(bytes.Buffer)
-	for _, p := range params {
-		buf.Write(ethutil.Encode(p))
-	}
-	return buf.Bytes()
+	p := bytes.NewReader(ethutil.Encode(params))
+	return Msg{Code: code, Size: uint32(p.Len()), Payload: p}
 }
 
 // Decode parse the RLP content of a message into
@@ -71,8 +60,7 @@ func encodePayload(params ...interface{}) []byte {
 //
 // For the decoding rules, please see package rlp.
 func (msg Msg) Decode(val interface{}) error {
-	s := rlp.NewListStream(msg.Payload, uint64(msg.Size))
-	if err := s.Decode(val); err != nil {
+	if err := rlp.Decode(msg.Payload, val); err != nil {
 		return newPeerError(errInvalidMsg, "(code %#x) (size %d) %v", msg.Code, msg.Size, err)
 	}
 	return nil