The SQL thread keeps track of the position in the current relay log from
which to read the next event. This position is not normally used, but a
certain interaction with the IO thread can cause the SQL thread to re-open
the relay log and seek to the stored position.

In parallel replication, there were a couple of places where the position
was not updated. This created a race where a re-open of the relay log could
seek to the wrong position and start re-reading and processing events
already handled once, causing various kinds of problems.

Fix this by moving the position update into a single place in
apply_event_and_update_pos(), which should ensure that the position is
always updated in the parallel replication case.

This problem was found from the testcase of MDEV-10863, but it is logically
a separate problem.