DROP USER 
RENAME USER CURRENT_USER() ...
GRANT ... TO CURRENT_USER()
REVOKE ... FROM CURRENT_USER()
ALTER DEFINER = CURRENT_USER() EVENTbut, When these statements are binlogged, CURRENT_USER() just is binlogged
as 'CURRENT_USER()', it is not expanded to the real user name. When slave 
executes the log event, 'CURRENT_USER()' is expand to the user of slave 
SQL thread, but SQL thread's user name always NULL. This breaks the replication.

After this patch, session's user will be written into query log events 
if these statements call CURREN_USER() or 'ALTER EVENT' does not assign a definer.


mysql-test/include/diff_tables.inc:
  Expend its abilities.
  Now it can diff not only in sessions of 'master' and 'slave', but 
  other sessions as well.
mysql-test/include/rpl_diff_tables.inc:
  Diff the same table between master and slaves.
sql/log_event.cc:
  session's user will be written into Query_log_event, if is_current_user_used() is TRUE.
  On slave SQL thread, Only thd->variables.current_user is written into Query_log_event,
  if it exists.
sql/sql_acl.cc:
  On slave SQL thread, grantor should copy from thd->variables.current_user, if it exists
sql/sql_class.h:
  On slave SQL thread, thd->variables.current_user is used to store the applying event's
  invoker.