"Grantor" columns' data is lost when replicating mysql.tables_priv.
Slave SQL thread used its default user ''@'' as the grantor of GRANT|REVOKE
statements executing on it.

In this patch, current user is put in query log event for all GRANT and REVOKE
statement, SQL thread uses the user in query log event as grantor.


mysql-test/suite/rpl/r/rpl_do_grant.result:
  Add test for this bug.
mysql-test/suite/rpl/t/rpl_do_grant.test:
  Add test for this bug.
sql/log_event.cc:
  Refactoring THD::current_user_used and related functions.
  current_user_used is used to judge if current user should be
  binlogged in query log event. So it is better to call it m_binlog_invoker.
  The related functions are renamed too.
sql/sql_class.cc:
  Refactoring THD::current_user_used and related functions.
  current_user_used is used to judge if current user should be
  binlogged in query log event. So it is better to call it m_binlog_invoker.
  The related functions are renamed too.
sql/sql_class.h:
  Refactoring THD::current_user_used and related functions.
  current_user_used is used to judge if current user should be
  binlogged in query log event. So it is better to call it m_binlog_invoker.
  The related functions are renamed too.
sql/sql_parse.cc:
  Call binlog_invoker() for GRANT and REVOKE statements.