Reimplement SubscriptionRef in Terms of ZHub

[adamgfraser]

One issue with the current implementation of SubscriptionRef is that since it is backed by a Queue, if more than one subscriber is observing changes to the Ref each subscriber will miss changes, since each time a value is taken from the queue by one subscriber it is no longer available to other subscribers. We can address this by backing SubscriptionRef with a Hub . This way each subscriber can receive the current value of the Ref and all changes after they have subscribed.

This allows SubscriptionRef to function much like a Var in functional reactive programming frameworks, where the SubscriptionRef represents some state that can be changes and multiple processes can subscribe to take actions in response to any changes in that state.

[iravid]

This is awesome.