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This patch makes the relationship between Event_scheduler and Event_queue
unidirectional from the former to the latter.




The change is that the conditional on which the scheduler sleeped has been
moved to the Event_queue and the latter does not call anymore
Event_scheduler::queue_changed(), which in turn has be removed.


sql/event_queue.cc:
  Remove dependency of Event_queue on Event_scheduler but not vice versa.
  Event_scheduler polls whether there is time to execute an event.
  
  Removed notify_observers() as the way of calling has changed.
  Added Event_queue::cond_wait() similar to Event_scheduler::cond_wait().
sql/event_queue.h:
  init_queue() does not need anymore Event_scheduler object because
  the relationship is now one-way. Event_scheduler knows about Event_queue
  but not vice versa.
  
  get_top_execution_if_time() does by itself the waiting instead of
  returning abstime. This simplifies the code in Event_scheduler::run()
  get_top_execution_i...