Correctly handle GTID event group for stand-alone multi-table ROW events

When a statement is binlogged stand-alone (as opposed to bracketed by
BEGIN/COMMIT or BEGIN/XID), the GTID code needs to correctly detect the end of
the event group without the availability of COMMIT or XID.

In row-based binlogging with multi-table updates, multiple row events can be
logged in a single stand-alone event group. The last such row event has a
special flag that marks the end of the group.

The GTID code must correctly detect and act on this flag (currently it does
not).