lnwire: use write buffer in Encode methods

This commit changes the WriteElement and WriteElements methods to take a
write buffer instead of io.Writer. The corresponding Encode methods are
changed to use the write buffer.

lnwire/onion_error_test.go

@@ -275,6 +275,8 @@ func (f *mockFailIncorrectDetailsNoHeight) Decode(r io.Reader, pver uint32) erro
 	return nil
 }
 
-func (f *mockFailIncorrectDetailsNoHeight) Encode(w io.Writer, pver uint32) error {
+func (f *mockFailIncorrectDetailsNoHeight) Encode(w *bytes.Buffer,
+	pver uint32) error {
+
 	return WriteElement(w, f.amount)
 }