Replace Revision.Spec.Active with RevisionServingState Enum (#213)
Replace Revision.Spec.Active with RevisionServingState Enum (#213)

The ServingState has three types:

Active: Ready to serve traffic.
Reserve: Not currently serving traffic, but could be made to serve
         traffic quickly.
Retired: No longer needed, resources can be torn down.
knative/serving,knative/serving