grandpa: fix handling of catch-up requests (#3956)

* grandpa: fix handling of catch-up requests

* grandpa: fix tests

* grandpa: add test for catch-up handling when observer disabled

* grandpa: extend doc comment

* grandpa: rename existing catch up test

substrate/node-template/src/service.rs

@@ -125,6 +125,8 @@ pub fn new_full<C: Send + Default + 'static>(config: Configuration<C, GenesisCon
 		justification_period: 512,
 		name: Some(name),
 		keystore: Some(service.keystore()),
+		observer_enabled: true,
+		is_authority,
 	};
 
 	match (is_authority, disable_grandpa) {