- fixed silly bug, the main thread restarted but did not execute events,
  Quite currious why many calls to pthread_mutex_init() do not lead to abort()


sql/event.cc:
  - remove mysql_event_table_exists
  - fix possible crash when table is 0x0
sql/event_executor.cc:
  - make event_executor_running_global_var volatile
  - fix erroneous reinitilization of a mutex, why did it not crash in debug mode?
    why pthread_mutex_init() does not abort() in case the mutex was not deinitted
    beforehand?
  - first initialization of event_executor_running_global_var inside init_mutexes()
  - remove debug if()
sql/event_priv.h:
  - remove unneeded definitions
sql/event_timed.cc:
  make backup and then restore the open table state of thd