lnwire: add new closing_complete and closing_sig messages

These two messages will be used to implement the new and improved co-op
closing protocol. This PR also show cases how to use the new
`tlv.OptionalRecord` type to define and handle TLV level parsing.

I think we can make one additional helper function to clean up some of
the boiler plate for the encode/decode.

lnwire/closing_sig.go

@@ -0,0 +1,76 @@
+package lnwire
+
+import (
+	"bytes"
+	"io"
+)
+
+// ClosingSig is sent in response to a ClosingComplete message. It carries the
+// signatures of the closee to the closer.
+type ClosingSig struct {
+	// ChannelID serves to identify which channel is to be closed.
+	ChannelID ChannelID
+
+	// ClosingSigs houses the 3 possible signatures that can be sent.
+	ClosingSigs
+
+	// ExtraData is the set of data that was appended to this message to
+	// fill out the full maximum transport message size. These fields can
+	// be used to specify optional data such as custom TLV fields.
+	ExtraData ExtraOpaqueData
+}
+
+// Decode deserializes a serialized ClosingSig message stored in the passed
+// io.Reader.
+func (c *ClosingSig) Decode(r io.Reader, _ uint32) error {
+	// First, read out all the fields that are hard coded into the message.
+	err := ReadElements(r, &c.ChannelID)
+	if err != nil {
+		return err
+	}
+
+	// With the hard coded messages read, we'll now read out the TLV fields
+	// of the message.
+	var tlvRecords ExtraOpaqueData
+	if err := ReadElements(r, &tlvRecords); err != nil {
+		return err
+	}
+
+	if err := decodeClosingSigs(&c.ClosingSigs, tlvRecords); err != nil {
+		return err
+	}
+
+	if len(tlvRecords) != 0 {
+		c.ExtraData = tlvRecords
+	}
+
+	return nil
+}
+
+// Encode serializes the target ClosingSig into the passed io.Writer.
+func (c *ClosingSig) Encode(w *bytes.Buffer, _ uint32) error {
+	if err := WriteChannelID(w, c.ChannelID); err != nil {
+		return err
+	}
+
+	recordProducers := closingSigRecords(&c.ClosingSigs)
+
+	err := EncodeMessageExtraData(&c.ExtraData, recordProducers...)
+	if err != nil {
+		return err
+	}
+
+	return WriteBytes(w, c.ExtraData)
+}
+
+// MsgType returns the uint32 code which uniquely identifies this message as a
+// ClosingSig message on the wire.
+//
+// This is part of the lnwire.Message interface.
+func (c *ClosingSig) MsgType() MessageType {
+	return MsgClosingSig
+}
+
+// A compile time check to ensure ClosingSig implements the lnwire.Message
+// interface.
+var _ Message = (*ClosingSig)(nil)