grandpa: observer doesn't send catch up messages (#3460)

* grandpa: don't send catch up requests when running GRANDPA observer

* grandpa: fix tests

* grandpa: add tests for catch up requests

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

@@ -1215,6 +1215,7 @@ fn voter_persists_its_votes() {
 			config.clone(),
 			set_state,
 			Exit,
+			true,
 		);
 		runtime.block_on(routing_work).unwrap();