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.

server.go

@@ -451,7 +451,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 	var (
 		err           error
 		nodeKeyECDH   = keychain.NewPubKeyECDH(*nodeKeyDesc, cc.KeyRing)
-		nodeKeySigner = keychain.NewPubKeyDigestSigner(
+		nodeKeySigner = keychain.NewPubKeyMessageSigner(
 			*nodeKeyDesc, cc.KeyRing,
 		)
 	)