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/observer.rs

@@ -260,7 +260,7 @@ where
 			&voters,
 			self.client.clone(),
 			&self.network,
-			&self.keystore,
+			self.keystore.as_ref(),
 			None,
 		);