the parsing. Next step will be to refactor of usage of Event_timed 
during Events::create_event() and Events::update_event().

Disallow:
- CREATE EVENT ... DO CREATE EVENT ...;
- ALTER  EVENT ... DO CREATE EVENT ...;
- CREATE EVENT ... DO ALTER EVENT DO ....;
- CREATE PROCEDURE ... BEGIN CREATE EVENT ... END|

Allowed:
- CREATE EVENT ... DO DROP EVENT yyy;
- CREATE EVENT ... DO ALTER EVENT yyy;
  (the nested ALTER EVENT can have anything but DO clause)
- ALTER  EVENT ... DO ALTER EVENT yyy;
  (the nested ALTER EVENT can have anything but DO clause)
- ALTER  EVENT ... DO DROP EVENT yyy;
- CREATE PROCEDURE ... BEGIN ALTER EVENT ... END|
  (the nested ALTER EVENT can have anything but DO clause)
- CREATE PROCEDURE ... BEGIN DROP EVENT ... END|