This will hopefully fix events.test failure on vmware-win32, 
where scheduling is very primitive.


mysql-test/t/events_scheduling.test:
  This test case has no races now and can be enabled under valgrind.
sql/event_scheduler.cc:
  Make Event Scheduler thread shutdown more synchronous: report
  successful shutdown only after having removed the scheduler
  thread from the global list of threads.
  This ensures that after the scheduler has been stopped, its
  thread is not present in SHOW PROCESSLIST output.