channeldb: replace raw keys in ChannelConfig with keychain.KeyDescriptor

In this commit, we remove references to raw keys from the main
ChannelConfig struct and instead replace it with usage of
keychain.KeyDescriptor. We do this, as the ChannelConfig as it stands
is a near complete static description of a channel. In the future, it
will be possible to export these static descriptions as backups. We
prefer the KeyDescriptor of a plain PublicKey, as the KeyLocator
portion of the struct allows a stateless signer to re-derive the keys
as needed when signing.

channeldb/codec.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 
+	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/shachain"
 	"github.com/roasbeef/btcd/btcec"
@@ -48,6 +49,22 @@ func readOutpoint(r io.Reader, o *wire.OutPoint) error {
 // to dynamically expand to accommodate additional data.
 func writeElement(w io.Writer, element interface{}) error {
 	switch e := element.(type) {
+	case keychain.KeyDescriptor:
+		if err := binary.Write(w, byteOrder, e.Family); err != nil {
+			return err
+		}
+		if err := binary.Write(w, byteOrder, e.Index); err != nil {
+			return err
+		}
+
+		if e.PubKey != nil {
+			if err := binary.Write(w, byteOrder, true); err != nil {
+			}
+
+			return writeElement(w, e.PubKey)
+		}
+
+		return binary.Write(w, byteOrder, false)
 	case ChannelType:
 		if err := binary.Write(w, byteOrder, e); err != nil {
 			return err
@@ -163,6 +180,23 @@ func writeElements(w io.Writer, elements ...interface{}) error {
 // encoded using the serialization format of the database.
 func readElement(r io.Reader, element interface{}) error {
 	switch e := element.(type) {
+	case *keychain.KeyDescriptor:
+		if err := binary.Read(r, byteOrder, &e.Family); err != nil {
+			return err
+		}
+		if err := binary.Read(r, byteOrder, &e.Index); err != nil {
+			return err
+		}
+
+		var hasPubKey bool
+		if err := binary.Read(r, byteOrder, &hasPubKey); err != nil {
+			return err
+		}
+
+		if hasPubKey {
+			return readElement(r, &e.PubKey)
+		}
+
 	case *ChannelType:
 		if err := binary.Read(r, byteOrder, e); err != nil {
 			return err