Track effect of an object on dependent children

[nilmerg]

Is your feature request related to a problem? Please describe.

In Icinga DB Web we'd like to show an indicator in lists showing the number of potentially affected children of a particular host/service. All affected children, i.e. also grandchildren.

Describe the solution you'd like

Icinga must calculate this for each parent during startup. Replace startup with whatever you like, my expectation is just that Icinga does not need to calculate this on every state change.

Though, what I'd like Icinga to calculate on every state change, is whether a parent is responsible for any now unreachable child, wherever in the hierarchy. (Somewhat similar to #10143)

The result should be that in the database the number is available in e.g. host.affected_children (uint) and host_state.affects_children (bool enum).