multi: use key locator for lnwallet.MessageSigner

To simplify the message signing API even further, we refactor the
lnwallet.MessageSigner interface to use a key locator instead of the
public key to identify which key should be signed with.

keychain/derivation.go

@@ -211,6 +211,10 @@ type SingleKeyMessageSigner interface {
 	// PubKey returns the public key of the wrapped private key.
 	PubKey() *btcec.PublicKey
 
+	// KeyLocator returns the locator that describes the wrapped private
+	// key.
+	KeyLocator() KeyLocator
+
 	// SignMessage signs the given message, single or double SHA256 hashing
 	// it first, with the wrapped private key.
 	SignMessage(message []byte, doubleHash bool) (*btcec.Signature, error)