WL#3337 (Event scheduler new architecture)

Post-review fixes. Mostly whitespace, int-to-bool return value, fixed comments


sql/Makefile.am:
  compile all submodules of Events before compiling the facade
sql/event_data_objects.cc:
  - Use initialization list
  - Clean whitespaces
  - Shorten comments
  - Fix comments
sql/event_data_objects.h:
  - Fix whitespace
sql/event_db_repository.cc:
  - Change return type from int to bool where only one error code is
    returned.
  - Don't use macros but get the maximal number of characters in a column
    from the column
  - Fix  comments
  - Make functions which has return value but it's not used - void.
sql/event_db_repository.h:
  - Methods with only one error code int -> bool return value
  - Remove declaration of fill_schema_events, a function that does not exist
sql/event_queue.cc:
  - Use initialization lists
  - Let find_n_remove_event delete the object thus making the code more robust.
    The caller could forget to destruct the object. In addition, find_n_remove_element()
    does not return a value.
  - Move check_system_tables() to class Events
  - Fix comments
sql/event_queue.h:
  - Whitespace changes
  - init_queue() should allow passing of THD
  - check_system_tables moved to class Events
  - find_n_remove_event() is now void
sql/event_scheduler.cc:
  - Initialize res before use
  - Remove end stop from message
sql/event_scheduler.h:
  Add uninitialized state. The scheduler is in it before init_scheduler()
  is called. The rationale is that otherwise state has no value before
  the call. If the system tables were damaged the scheduler won't be initialized
  but in Events::deinit() Event_scheduler::stop() will be called and this will
  touch state, generating valgrind warning at minimum.
sql/events.cc:
  - Whitespace changes
  - Fix comments
  - Make methods which have only one error code be bool instead of int
  - Create temporarily a THD to be used for the initialization of Event_queue
  - Event_queue::check_system_tables() moved to Events::check_system_tables
  - is_started() is renamed to is_execution_of_events_started()
sql/events.h:
  - Whitespace changes
  - When a method returns only one error code it should be bool, not int
  - is_started() renamed to is_execution_of_events_started()
sql/set_var.cc:
  is_started() is renamed to is_execution_of_events_started()
sql/sql_db.cc:
  The return code is not used, thus don't return anything and drop_schema_events()
  is now void.
sql/sql_yacc.yy:
  - Fix comments
  - Remove unneeded initialization which is performed in lex_init()
sql/share/errmsg.txt:
  New error message
sql/table.cc:
  - Fix comments
  - make table_check_intact() accespt const *table_def
sql/table.h:
  Make table_check_intact() accespt const *table_def

sql/sql_db.cc

@@ -949,7 +949,7 @@ bool mysql_rm_db(THD *thd,char *db,bool if_exists, bool silent)
 
 exit:
   (void)sp_drop_db_routines(thd, db); /* QQ Ignore errors for now  */
-  error= Events::get_instance()->drop_schema_events(thd, db);
+  Events::get_instance()->drop_schema_events(thd, db);
   /*
     If this database was the client's selected database, we silently
     change the client's selected database to nothing (to have an empty