Close subscription streams on remove

[FZambia]

This allows exiting from async stream iterators when subscription is removed from Client's registry (i.e. when Client.removeSubscription is called):

final subscription = client.newSubscription(...);

final onPublicationEvent = () async {
  await for (var i in subscription.publication) {
    print('publication: $i');
  }
  print("subscription removed");
};
onPublicationEvent();
client.removeSubscription(subscription)

Adresses #65

[FZambia]

Thx for looking - I'll try to merge it this week then