Added ignorable event type, binary log events of this class or its
subclasses will be silently ignored when applying by the slave SQL
thread if it can not recognize them.
This patch also added a rows query log event type, which is a subclass
of the ignorable log event, to record the original query for the rows
events in RBR. This event can be used to display the original query as
comments by SHOW BINLOG EVENTS query, or mysqlbinlog client when the
--verbose option is given twice.