multi: refactor SignDigestCompact into SignMessageCompact

To make it possible to use a remote lnrpc server as a signer for our
wallet, we need to change our main interface to sign the message instead
of the message's digest. Otherwise we'd need to alter the
lnrpc.SignMessage RPC to accept a digest instead of only the message
which has security implications.

netann/channel_update_test.go

@@ -32,7 +32,7 @@ var _ lnwallet.MessageSigner = (*mockSigner)(nil)
 
 var (
 	privKey, _    = btcec.NewPrivateKey(btcec.S256())
-	privKeySigner = &keychain.PrivKeyDigestSigner{PrivKey: privKey}
+	privKeySigner = &keychain.PrivKeyMessageSigner{PrivKey: privKey}
 
 	pubKey = privKey.PubKey()