handleSubscription should unregister itself on close

[aschmahmann]

If for some reason the subscription dies or gets cancelled without being managed by PubsubValueStore.Cancel(key) then we can't resubscribe until we first call PubSubValueStore.Cancel(key).

go-libp2p-pubsub-router/pubsub.go

Lines 290 to 292 in 1b619f8

	func (p *PubsubValueStore) handleSubscription(sub *pubsub.Subscription, key string, cancel func()) {
	defer sub.Cancel()
	defer cancel()

If instead of just cancelling the subscription as above we triggered an internal cancellation function that would likely be better.

[aschmahmann]

closed by #37