pkg/health doesn't properly implement K8s lifecycle

[mattmoor]

Describe the bug
I noticed that eventing has:
https://github.com/knative/eventing/blob/master/pkg/health/probes.go

This blindly responds to liveness and readiness probes with "200 OK", which is fine for things coming up, but it doesn't properly implement lame ducking, which is critical to avoid dropped requests during scale down or rollouts.

Expected behavior

We have libraries for handling kubelet probes in PKG, use those and extend them as appropriate: https://github.com/knative/pkg/blob/d5c09d2aef18eaabbb9ce3ac71a487288ca542aa/network/handlers/drain.go#L78

cc @pierDipi @vaikas @tcnghia