MDEV-5769: Slave crashes on attempt to do parallel replication from an older master

Older master has no GTID events, so such events are not available for
deciding on scheduling of event groups and so on.

With this patch, we run such events from old masters single-threaded, in the
sql driver thread.

This seems better than trying to make the parallel code handle the data from
older masters; while possible, this would require a lot of testing (as well as
possibly some extra overhead in the scheduling of events), which hardly seems
worthwhile.

mysql-test/suite/rpl/r/rpl_old_master.result

@@ -0,0 +1,27 @@
+include/master-slave.inc
+[connection master]
+include/stop_slave.inc
+include/rpl_stop_server.inc [server_number=1]
+include/rpl_start_server.inc [server_number=1]
+SET @old_parallel= @@GLOBAL.slave_parallel_threads;
+SET GLOBAL slave_parallel_threads=10;
+CHANGE MASTER TO master_host='127.0.0.1', master_port=SERVER_MYPORT_1, master_user='root', master_log_file='master-bin.000001', master_log_pos=4;
+include/start_slave.inc
+CREATE TABLE t2 (a INT PRIMARY KEY) ENGINE=InnoDB;
+INSERT INTO t2 VALUES (1);
+SELECT * FROM t1 ORDER BY a;
+a	b
+1	1
+2	2
+3	4
+4	8
+5	16
+SELECT * FROM t2;
+a
+1
+include/stop_slave.inc
+SET GLOBAL slave_parallel_threads=@old_parallel;
+DROP TABLE t1;
+include/start_slave.inc
+DROP TABLE t2;
+include/rpl_end.inc