suppress only normal notifications from service parents

[sni]

This change makes service parents work more like service dependencies. Previously service parents simply dropped all notifications which leads to some issues:

• if a service goes down, sends a notification but comes up before the parent, no recover will be sent
• forced notifications, downtimes etc won't send a notification

With this change only "normal" notifications will be suppressed if all parents are down. But the following notifications will still be sent:

• custom notifications
• acknowlegements
• flapping alerts
• downtime alerts