An attempt to fix a sporadic valgrind memory leak in Event Scheduler:

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/sql_class.h

@@ -656,6 +656,18 @@ public:
   }
   
   bool set_user(char *user_arg);
+
+#ifndef NO_EMBEDDED_ACCESS_CHECKS
+  bool
+  change_security_context(THD *thd,
+                          LEX_STRING *definer_user,
+                          LEX_STRING *definer_host,
+                          LEX_STRING *db,
+                          Security_context **backup);
+
+  void
+  restore_security_context(THD *thd, Security_context *backup);
+#endif
 };