BIP0144+wire: add a MessageEncoding variant for serialization/deserialization

This commit modifies the existing wire.Message interface to introduce a
new MessageEncoding variant which dictates the exact encoding to be
used when serializing and deserializing messages. Such an option is now
necessary due to the segwit soft-fork package, as btcd will need to be
able to optionally encode transactions/blocks without witness data to
un-upgraded peers.

Two new functions have been introduced: ReadMessageWithEncodingN and
WriteMessageWithEncodingN which wrap BtcDecode/BtcEncode with the
desired encoding format.

wire/msgverack_test.go

@@ -41,10 +41,11 @@ func TestVerAckWire(t *testing.T) {
 	msgVerAckEncoded := []byte{}
 
 	tests := []struct {
-		in   *MsgVerAck // Message to encode
-		out  *MsgVerAck // Expected decoded message
-		buf  []byte     // Wire encoding
-		pver uint32     // Protocol version for wire encoding
+		in   *MsgVerAck      // Message to encode
+		out  *MsgVerAck      // Expected decoded message
+		buf  []byte          // Wire encoding
+		pver uint32          // Protocol version for wire encoding
+		enc  MessageEncoding // Message encoding format
 	}{
 		// Latest protocol version.
 		{
@@ -52,6 +53,7 @@ func TestVerAckWire(t *testing.T) {
 			msgVerAck,
 			msgVerAckEncoded,
 			ProtocolVersion,
+			BaseEncoding,
 		},
 
 		// Protocol version BIP0035Version.
@@ -60,6 +62,7 @@ func TestVerAckWire(t *testing.T) {
 			msgVerAck,
 			msgVerAckEncoded,
 			BIP0035Version,
+			BaseEncoding,
 		},
 
 		// Protocol version BIP0031Version.
@@ -68,6 +71,7 @@ func TestVerAckWire(t *testing.T) {
 			msgVerAck,
 			msgVerAckEncoded,
 			BIP0031Version,
+			BaseEncoding,
 		},
 
 		// Protocol version NetAddressTimeVersion.
@@ -76,6 +80,7 @@ func TestVerAckWire(t *testing.T) {
 			msgVerAck,
 			msgVerAckEncoded,
 			NetAddressTimeVersion,
+			BaseEncoding,
 		},
 
 		// Protocol version MultipleAddressVersion.
@@ -84,6 +89,7 @@ func TestVerAckWire(t *testing.T) {
 			msgVerAck,
 			msgVerAckEncoded,
 			MultipleAddressVersion,
+			BaseEncoding,
 		},
 	}
 
@@ -91,7 +97,7 @@ func TestVerAckWire(t *testing.T) {
 	for i, test := range tests {
 		// Encode the message to wire format.
 		var buf bytes.Buffer
-		err := test.in.BtcEncode(&buf, test.pver)
+		err := test.in.BtcEncode(&buf, test.pver, test.enc)
 		if err != nil {
 			t.Errorf("BtcEncode #%d error %v", i, err)
 			continue
@@ -105,7 +111,7 @@ func TestVerAckWire(t *testing.T) {
 		// Decode the message from wire format.
 		var msg MsgVerAck
 		rbuf := bytes.NewReader(test.buf)
-		err = msg.BtcDecode(rbuf, test.pver)
+		err = msg.BtcDecode(rbuf, test.pver, test.enc)
 		if err != nil {
 			t.Errorf("BtcDecode #%d error %v", i, err)
 			continue