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/event_data_objects.cc:
  Introduced a new class Event_queue_element_for_exec
  According to Konstantin it should be named Event_name and hold
  only two LEX_STRINGs but `dropped` is not saved on disk and will
  require additional logic in Event_worker_thread class, after loading
  to compute whether the event should be dropped or not. It's easier
  just to pass this flag around.
  
  Removed Event_queue_element::drop(). This method was a source of a
  race condition. At the place where the event should be dropped we
  call Events::drop_event() which is the only code-flow for dropping.
  In addition, because ::drop_event() holds Events::LOCK_metadata there is
  no source of race conditions. Before this patch dropping from ::drop()
  wasn't under LOCK_metadata and races were possible.
  
  Because Events::open_event_table was removed as a method, provisionally
  events_event_db_repository was exported from events.cc till a solution is
  build where Event_queue_element does not access directly mysql.event.
sql/event_data_objects.h:
  New class Event_queue_element_for_exec added which is returned
  from Event_queue::get_top_if_time() and passed through Event_scheduler
  to Event_worker_thread. There by using the (db)name Event_job_data is
  instanciated and executed.
  
  Dropped Event_queue_element::drop()
  
  thd was moved out of Event_job_data as it is now part of
  Event_queue_element_for_exec
sql/event_queue.cc:
  Removed dependency of Event_queue on Event_db_repository.
  The instantiation of Event_job_data was moved to class Event_worker_thread
  In place is a return of an object of Event_queue_element_for_exec is used
  later for instantiating Event_job_data.
  
  The `dropped` flag of Event_queue_element is passed over
  Event_queue_element_for_exec to the code in Event_worker_thread.
sql/event_queue.h:
  Removed dependency of Event_queue on Event_db_repository
  Removed dependency on Event_scheduler
sql/event_scheduler.cc:
  Added class Event_worker_thread, which is used during
  the execution of an event. It has a static init() method
  to get a pointer to Event_db_repository to be used for
  instantiation of Event_job_data object. This object it then
  executed.
sql/event_scheduler.h:
  Added class Event_worker_thread, which is used during
  the execution of an event.
sql/events.cc:
  Removed Events::open_event_table() because it was a product of
  a bad architecture.
sql/events.h:
  Removed friend definition, unneeded.
  
  Fixed Events::drop_event() to have the previous signature without
  bool only_from_disk
sql/sql_parse.cc:
  Fix call

sql/event_queue.h

@@ -16,12 +16,10 @@
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 
 class Event_basic;
-class Event_db_repository;
-class Event_job_data;
 class Event_queue_element;
+class Event_queue_element_for_exec;
 
 class THD;
-class Event_scheduler;
 
 class Event_queue
 {
@@ -35,19 +33,19 @@ public:
   deinit_mutexes();
 
   bool
-  init_queue(THD *thd, Event_db_repository *db_repo);
+  init_queue(THD *thd);
   
   void
   deinit_queue();
 
   /* Methods for queue management follow */
 
-  int
-  create_event(THD *thd, LEX_STRING dbname, LEX_STRING name);
+  void
+  create_event(THD *thd, Event_queue_element *new_element);
 
-  int
+  void
   update_event(THD *thd, LEX_STRING dbname, LEX_STRING name,
-               LEX_STRING *new_schema, LEX_STRING *new_name);
+               Event_queue_element *new_element);
 
   void
   drop_event(THD *thd, LEX_STRING dbname, LEX_STRING name);
@@ -59,14 +57,15 @@ public:
   recalculate_activation_times(THD *thd);
 
   bool
-  get_top_for_execution_if_time(THD *thd, Event_job_data **job_data);
+  get_top_for_execution_if_time(THD *thd,
+                                Event_queue_element_for_exec **event_name);
+
 
   void
   dump_internal_status();
 
-  int
-  load_events_from_db(THD *thd);
-
+  void
+  empty_queue();
 protected:
   void
   find_n_remove_event(LEX_STRING db, LEX_STRING name);
@@ -76,8 +75,6 @@ protected:
   drop_matching_events(THD *thd, LEX_STRING pattern,
                        bool (*)(LEX_STRING, Event_basic *));
 
-  void
-  empty_queue();
 
   void
   dbug_dump_queue(time_t now);
@@ -86,11 +83,7 @@ protected:
   pthread_mutex_t LOCK_event_queue;
   pthread_cond_t COND_queue_state;
 
-  Event_db_repository *db_repository;
-
-  Event_scheduler *scheduler;
-
-  /* The sorted queue with the Event_job_data objects */
+  /* The sorted queue with the Event_queue_element objects */
   QUEUE queue;
 
   TIME next_activation_at;