added new exported unregister session call

[brobits]

useful for test cleanup in other packages. an example, when SessionIDs are tracked in sessions:

for _, s := range sessions {
	err := fix.UnregisterSession(s)
	if err != nil {
		log.Printf("could not remove session %s: %s", s.String(), err.Error())
	}
}

[cbusbey]

Thanks bits 🍻