More small fixes to the API : use LEX_STRING instead of LEX_STRING* and if error
then return bool(true) instead of error code.
Merged functions. Reduced usage of sp_name.
Fixed a lot of function documentation errors.
Added function documentation wherever needed.
Removed some unused defines and error codes.

Next to come is batch rename of Event_scheduler_ng to Event_scheduler.


mysql-test/r/events.result:
  update result
mysql-test/r/events_logs_tests.result:
  update result
mysql-test/t/events.test:
  more test coverage
mysql-test/t/events_logs_tests.test:
  fix test
sql/event_data_objects.cc:
  Cosmetics.
  Fix function documentation whenever needed.
  Move Event_job_data::compile() next to Event_job_data::execute()
sql/event_data_objects.h:
  Remove unneeded error codes and defines
  Move function declarations at the end of the header
sql/event_db_repository.cc:
  Fix function documentation.
  Event_db_repository::update_event() now uses LEX_STRING *-s instead of
  sp_name . Lower coupling.
sql/event_db_repository.h:
  Event_db_repository::update_event() now uses LEX_STRING *-s instead of
  sp_name . Lower coupling.
  find_event -> find_named_event
  find_event_by_name is not used externally, merge with load_named_event()
sql/event_queue.cc:
  LEX_STRING* to LEX_STRING
  Fix comments.
  Fix and add function documentation.
  Remove Event_queue::events_count() as it is unused
  Change get_top_for_execution_if_time() to return status code as return value
  and the object is in out parameter.
sql/event_queue.h:
  LEX_STRING* to LEX_STRING
  Fix comments.
  Fix and add function documentation.
  Remove Event_queue::events_count() as it is unused
  Change get_top_for_execution_if_time() to return status code as return value
  and the object is in out parameter.
  Try to detect also lock attemptions for deadlocks.
sql/event_scheduler_ng.cc:
  Always execute on thd->mem_root
  Fix according to changed API of Event_queue::get_top_for_execution_if_time()
sql/events.cc:
  Fix function documentation.
  Fix code after API changes of internal Event module classes.
sql/events.h:
  sp_name -> LEX_STRINGs
sql/sql_parse.cc:
  Fix according to changed API of Events::show_create_event()
sql/sql_yacc.yy:
  Don't pass NULL as third parameter to sp_head::init_strings()