chainHead subscriptions aren't cleaned up properly

[tomaka]

Problem introduced in #793

The subscriptions are never cleaned up. By itself this isn't a problem, but then at some point the limit to the number of subscriptions will be reached.

[tomaka]

After some thinking, I think that the best course of action is to generate a SubscriptionDestroyed event (from run_until_event) once a Subscription is destroyed. This would let the API user do some local clean up.

The client main task should also only ever count a subscription as unsubscribed after the Subscription object is destroyed, rather than immediately.