Follow-up patch, fixing a possible deadlock issue.

If the master crashes in the middle of an event group, there can be an active
transaction in a worker thread when we encounter the following master restart
format description event. In this case, we need to notify that worker thread
to abort and roll back the partial event group. Otherwise a deadlock occurs:
the worker thread waits for the commit that never arrives, and the SQL driver
thread waits for the worker thread to complete its event group, which it never
does.