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.

lnrpc/lightning.proto

@@ -1086,6 +1086,12 @@ message SignMessageRequest {
     base64.
     */
     bytes msg = 1;
+
+    /*
+    Instead of the default double-SHA256 hashing of the message before signing,
+    only use one round of hashing instead.
+    */
+    bool single_hash = 2;
 }
 message SignMessageResponse {
     // The signature for the given message