streamline the event worker thread work flow and try to eliminate
possibilities for memory corruptions that might have been
lurking in previous (complicated) code.
This patch: 
 * removes Event_job_data::compile that was never used
 * cleans up Event_job_data::execute to minimize juggling with
   thread context and eliminate unneded code paths
 * Implements Security_context::change/restore_security_context
   to be able to re-use these methods in all stored programs
This is to maybe fix Bug#27733 "Valgrind failures in 
remove_table_from_cache".
Review comments applied.


sql/event_data_objects.cc:
  Remove Event_job_data::compile, which was never used without
  Event_job_data::execute().
  Merge the implementation of compile() with Event_job_data::execute().
  Reuse existing functions to prepare the event worker thread
  for execution instead of some previously copy-pasted code.
  Do not change and restore the current database inside 
  Event_job_data::execute(), just set the current database in the 
  thread, that is enough to parse and execute an event.
sql/event_data_objects.h:
  Update declarations.
sql/event_scheduler.cc:
  Allocate Event_job_data on stack.
sql/item_func.cc:
  Update to match the new declaration of restore_security_context()
sql/sp_head.cc:
  Update to match the new declaration of 
  change/restore_security_context()
sql/sql_class.cc:
  Move change/restore_security_context to class Security_context.
  Add more comments.
sql/sql_class.h:
  Make change/restore_security_context methods of Security_context.
  That allows us to reuse them in Event Scheduler (instead of a
  copy-paste presently used there).
sql/sql_trigger.cc:
  Update to match the new declaration of 
  change/restore_security_context()