MDEV-5764: START SLAVE UNTIL does not work with parallel replication

With parallel replication, there can be any number of events queued on
in-memory lists in the worker threads.

For normal STOP SLAVE, we want to skip executing any remaining events on those
lists and stop as quickly as possible.

However, for START SLAVE UNTIL, when the UNTIL position is reached in the SQL
driver thread, we must _not_ stop until all already queued events for the
workers have been executed - otherwise we would stop too early, before the
actual UNTIL position had been completely reached.

The code did not handle UNTIL correctly, stopping too early due to not
executing the queued events to completion. Fix this, and also implement that
an explicit STOP SLAVE in the middle (when the SQL driver thread has reached
the UNTIL position but the workers have not) _will_ cause an immediate stop.

sql/rpl_rli.h

@@ -262,6 +262,7 @@ public:
   */
   volatile bool inited;
   volatile bool abort_slave;
+  volatile bool stop_for_until;
   volatile uint slave_running;
 
   /*