Third patch of the bug fix where the code for skipping events and for
executing events is factored out into three functions:
- shall_skip() to decide if the event shall be skipped and the
  reason for it;
- do_apply_event(), where the event is applied to the database; and
- do_update_pos(), which updates the actual relay log position and
  group positions.


mysql-test/r/rpl_row_tabledefs_2myisam.result:
  Result change.
mysql-test/r/rpl_row_tabledefs_3innodb.result:
  Result change.
sql/log_event.cc:
  Creating shall_skip(), do_update_pos(), and do_apply_event()
  functions for each event by factoring out the previous code.
  Adding debug code and fixing some error codes that were not correct.
sql/rpl_rli.cc:
  Renaming unsafe_to_stop_at into last_event_start_time.
  Adding debug code.
sql/rpl_rli.h:
  Renaming unsafe_to_stop_at into last_event_start_time.
sql/slave.cc:
  Renaming unsafe_to_stop_at into last_event_start_time.