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/cli/src/service.rs

@@ -177,6 +177,8 @@ macro_rules! new_full {
 			justification_period: 512,
 			name: Some(name),
 			keystore: Some(service.keystore()),
+			observer_enabled: true,
+			is_authority,
 		};
 
 		match (is_authority, disable_grandpa) {