Fix for bug#22740 Events: Decouple Event_queue from Event_db_repository

This patch implements the idea of the bug report by making Event_queue
unaware of Event_db_repository by making a higher level class - Events,
which is aware of most of all classes, responsible for passing all data
needed for adding/updating/deleting an event to/from the queue.

Introduces few new classes :
 - Event_worker_thread
 - Event_queue_element_for_exec

sql/sql_parse.cc

@@ -4047,8 +4047,7 @@ end_with_restore_list:
       if (!(res= Events::get_instance()->drop_event(thd,
                                                     lex->spname->m_db,
                                                     lex->spname->m_name,
-                                                    lex->drop_if_exists,
-                                                    FALSE)))
+                                                    lex->drop_if_exists)))
         send_ok(thd);
     }
     break;