grandpa: minor cleanups in communication module (#6371)

* grandpa: replace Result<(), ()> with Option<()>

* grandpa: replace &Option<T> with Option<&T>

* grandpa: cleanup local id and keystore usages

* grandpa: return bool on check_message_signature

* grandpa: fix erroneous log message on startup

* grandpa: fix test

substrate/client/finality-grandpa/src/tests.rs

@@ -1160,11 +1160,10 @@ fn voter_persists_its_votes() {
 		);
 
 		let (round_rx, round_tx) = network.round_communication(
-			Some(keystore),
+			Some((peers[1].public().into(), keystore).into()),
 			communication::Round(1),
 			communication::SetId(0),
 			Arc::new(VoterSet::new(voters).unwrap()),
-			Some(peers[1].public().into()),
 			HasVoted::No,
 		);