In parallel replication, there are two kinds of events which are
executed in different ways.
Normal events that are part of event groups/transactions are executed
asynchroneously by being queued for a worker thread.
Other events like format description and rotate and such are executed
directly in the driver SQL thread.
If the direct execution of the other events were to update the old-style
position, then the position gets updated too far ahead, before the normal
events that have been queued for a worker thread have been executed. So
this patch adds some special cases to prevent such position updates ahead
of time, and instead queues dummy events for the worker threads, so that
they will at an appropriate time do the position updates instead.
(Also fix a race in a test case that happened to trigger while running
tests for this patch).
Improve STOP SLAVE in parallel mode.
Now, the parallel part will queue the current event group to the
end, and then stop queing any more events. Each worker will
complete the current event group, and then just skip any further
queued events.
Wait for all worker threads to finish when stopping the SQL thread.
(Only a basic wait; this still needs to be fixed to include timeout
logic as in sql_slave_killed()).
