- improve the stability of the executor
- make create event if not exists work as before


sql/event.cc:
  refactoring:
  - have only 1 routine for comparing TIME structures 
  fix:
  - after previous refactoring IF NOT EXISTS of CREATE EVENT did not work 
    anymore. Now it will work.
sql/event.h:
  update definitions
sql/event_executor.cc:
  - don't load DISABLED events
  - if an event is being disabled because of time restrictions - drop it from the 
    prio queue
  - move dropping to the worker process
sql/event_priv.h:
  - remove unneeded func
sql/share/errmsg.txt:
  fix error message
sql/sql_parse.cc:
  - support 0 rows affected when CREATE EVENT IF NOT EXISTS