Final stroke, events should be loaded from disk on server startup.
Also check the validity of their bodies if possible during loading.


sql/event_data_objects.cc:
  Remove Event_job_data::free_sp(), move the code to the destructor
  Change the way we change the security context
  Steal some code from sql_parse.cc
sql/event_data_objects.h:
  Remove free_sp()
  Make compile() public, to be used when booting for verifying the integrity of mysql.event
sql/event_queue.cc:
  Make the queue load events from disk on server boot.
  Compile and thus check for integrity the events.
sql/event_queue.h:
  shift methods around. add queue_loaded boolean.
sql/event_scheduler.cc:
  Rename init_event_thread() to pre_init_event_thread()
  and make it more generic.
  Add post_init_event_thread()
  Export these two as well as deinit_event_thread().
  Now it is quite easy to write code to spawn a new event thread
  whenever needed.
sql/event_scheduler.h:
  export pre_init_event_thread(), post_init_event_thread() and deinit_event_thread()
  to simplify writing of thread functions.
sql/events.cc:
  Events::init() returns only one error code, then make it bool
sql/events.h:
  Events::init() returns only one error code, then make it bool
sql/mysqld.cc:
  Check the return code of Events::init()
sql/sp_head.cc:
  Add trace info
sql/sql_class.cc:
  Reorganize thd::change_security_context() to load main_security_ctx
sql/sql_class.h:
  Reorganize thd::change_security_context() to load main_security_ctx
sql/sql_lex.cc:
  Initialize lex->spname
sql/sql_yacc.yy:
  Add a comment