This cut No 7 should finish the part of fixing the parsing of the events :
- Event_timed is no more used during parsing. Less problems because it has
  a mutex. Event_parse_data class is used during parsing. It is suited only
  for this purpose. It's pretty lightweight
- Late checking of data from parsing is being performed. This should solve
  the problems of nested events in SP or other events (for the situation 
  of no nested bodies). Before if an ALTER EVENT was in a SP, then when the
  SP was compiled, and not executed, the actual init_xxx methods of Event_timed
  were called, which is wrong.
- It could be a side effect of using a specialized class, but test events_stress is
  now 25% quicker.

Cut No8 will start splitting Event_scheduler into 2 parts, the QUEUE will be moved
to Event_queue.