Support topic priorities
Some topics might be more important than others from a deployment perspective (e.g., a customer needs feature A before feature B). It should be possible to indicate that feature A has a higher priority and should always be staged prior to feature B.
On the policy side, this can be indicated by either a Topic-priority
trailer in the description (shaky), or based on a set of labels which have an order defined among them.
Cc: @brad.king