=act #3623 proposed impl for unsubscribing, when last channel subsc removed

* since we must check if the subscriber has any subscribed channels left
  we had to expose this detail from SubchannelClassification via
  `hasSubscriptions`. Increases cost of ubsubscribe(actor, channel) a bit.

Please review, @patriknw. Do you think adding this method in the
classification is justified by this need? Made it private[akka], don't
think many end-users would need this information.