Don't try to reset sync_standby_names if we never were in cluster.

Since for ALTER SYSTEM we reconnect to ourselves, we need the connstring and we
have no connstring we have never been in cluster.

Author: arssher

init.sql

@@ -316,7 +316,10 @@ BEGIN
 		-- otherwise we will hang
 		SET LOCAL synchronous_commit TO LOCAL;
 		-- TODO: remove only shardman's standbys
-		PERFORM shardman.set_sync_standbys('');
+		-- If we never were in the cluster, we didn't touch sync_standby_names
+		IF shardman.my_connstr() != NULL THEN
+			PERFORM shardman.set_sync_standbys('');
+		END IF;
 	END IF;
 
 	PERFORM shardman.reset_node_id();

membership.sql

@@ -99,15 +99,11 @@ BEGIN
 END
 $$ LANGUAGE plpgsql;
 
--- Get local node connstr. Throws an error, if node is not in cluster
+-- Get local node connstr. Returns NULL if node is not in cluster and never was
+-- in one.
 CREATE FUNCTION my_connstr() RETURNS text AS $$
-DECLARE
-	connstr text := connstring FROM shardman.nodes WHERE id = shardman.my_id();
 BEGIN
-	IF connstr IS NULL THEN
-		RAISE EXCEPTION '[SHMN] Node not in cluster, can''t get its connstring';
-	END IF;
-	RETURN connstr;
+	RETURN connstring FROM shardman.nodes WHERE id = shardman.my_id();
 END $$ LANGUAGE plpgsql;
 
 -- Get connstr of worker node with id node_id. ERROR is raised if there isn't