brontide/conn: migrate to WriteMessage + Flush

This commit modifies WriteMessage to only perform encryption on the
passed plaintext, and buffer the ciphertext within the connection
object. We then modify internal uses of WriteMessage to follow with a
call to Flush, which actually writes the message to the wire.
Additionally, since WriteMessage does not actually perform the write
itself, the io.Writer argument is removed from the function signature
and all call sites.

brontide/noise_test.go

@@ -220,11 +220,9 @@ func TestMaxPayloadLength(t *testing.T) {
 	// payload length.
 	payloadToReject := make([]byte, math.MaxUint16+1)
 
-	var buf bytes.Buffer
-
 	// A write of the payload generated above to the state machine should
 	// be rejected as it's over the max payload length.
-	err := b.WriteMessage(&buf, payloadToReject)
+	err := b.WriteMessage(payloadToReject)
 	if err != ErrMaxMessageLengthExceeded {
 		t.Fatalf("payload is over the max allowed length, the write " +
 			"should have been rejected")
@@ -233,7 +231,7 @@ func TestMaxPayloadLength(t *testing.T) {
 	// Generate another payload which should be accepted as a valid
 	// payload.
 	payloadToAccept := make([]byte, math.MaxUint16-1)
-	if err := b.WriteMessage(&buf, payloadToAccept); err != nil {
+	if err := b.WriteMessage(payloadToAccept); err != nil {
 		t.Fatalf("write for payload was rejected, should have been " +
 			"accepted")
 	}
@@ -243,7 +241,7 @@ func TestMaxPayloadLength(t *testing.T) {
 	payloadToReject = make([]byte, math.MaxUint16+1)
 
 	// This payload should be rejected.
-	err = b.WriteMessage(&buf, payloadToReject)
+	err = b.WriteMessage(payloadToReject)
 	if err != ErrMaxMessageLengthExceeded {
 		t.Fatalf("payload is over the max allowed length, the write " +
 			"should have been rejected")
@@ -502,10 +500,14 @@ func TestBolt0008TestVectors(t *testing.T) {
 	var buf bytes.Buffer
 
 	for i := 0; i < 1002; i++ {
-		err = initiator.WriteMessage(&buf, payload)
+		err = initiator.WriteMessage(payload)
 		if err != nil {
 			t.Fatalf("could not write message %s", payload)
 		}
+		_, err = initiator.Flush(&buf)
+		if err != nil {
+			t.Fatalf("could not flush message: %v", err)
+		}
 		if val, ok := transportMessageVectors[i]; ok {
 			binaryVal, err := hex.DecodeString(val)
 			if err != nil {