In inc_group_relay_log_pos, slave_timestamp is set to
rgi->last_master_timestamp, but for the serial slave,
rgi->last_master_timestamp is never set (as it was done
in the parallel-specific code), thereby never updating
slave_timestamp in this part. So to fix this, when the
serial slave reads a new group event, it updates
serial_rgi->last_master_timestamp so slave_timestamp
can be updated at transaction commit.

Additionally, the update to slave_timestamp in
Xid_apply_log_event::do_apply_event() was removed, as
it is not safe for the parallel slave, and is
redundant with the update in inc_group_relay_log_pos.