channeldb+htlcswitch: write wire messages using length prefix

In this commit, we modify the way we write wire messages across the
entire database. We'll now ensure that we always write wire messages
with a length prefix. We update the `codec.go` file to always write a 2
byte length prefix, this affects the way we write the `CommitDiff` and
`LogUpdates` struct to disk, and the network results bucket in the
switch as it includes a wire message.

channeldb/codec.go

@@ -1,6 +1,7 @@
 package channeldb
 
 import (
+	"bytes"
 	"encoding/binary"
 	"fmt"
 	"io"
@@ -178,7 +179,17 @@ func WriteElement(w io.Writer, element interface{}) error {
 		}
 
 	case lnwire.Message:
-		if _, err := lnwire.WriteMessage(w, e, 0); err != nil {
+		var msgBuf bytes.Buffer
+		if _, err := lnwire.WriteMessage(&msgBuf, e, 0); err != nil {
+			return err
+		}
+
+		msgLen := uint16(len(msgBuf.Bytes()))
+		if err := WriteElements(w, msgLen); err != nil {
+			return err
+		}
+
+		if _, err := w.Write(msgBuf.Bytes()); err != nil {
 			return err
 		}
 
@@ -394,7 +405,13 @@ func ReadElement(r io.Reader, element interface{}) error {
 		*e = bytes
 
 	case *lnwire.Message:
-		msg, err := lnwire.ReadMessage(r, 0)
+		var msgLen uint16
+		if err := ReadElement(r, &msgLen); err != nil {
+			return err
+		}
+
+		msgReader := io.LimitReader(r, int64(msgLen))
+		msg, err := lnwire.ReadMessage(msgReader, 0)
 		if err != nil {
 			return err
 		}