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.

peer/test_utils.go

@@ -61,7 +61,7 @@ func createTestPeer(notifier chainntnfs.ChainNotifier,
 	aliceKeyPriv, aliceKeyPub := btcec.PrivKeyFromBytes(
 		btcec.S256(), channels.AlicesPrivKey,
 	)
-	aliceKeySigner := &keychain.PrivKeyDigestSigner{PrivKey: aliceKeyPriv}
+	aliceKeySigner := &keychain.PrivKeyMessageSigner{PrivKey: aliceKeyPriv}
 	bobKeyPriv, bobKeyPub := btcec.PrivKeyFromBytes(
 		btcec.S256(), channels.BobsPrivKey,
 	)