Client.subscribe breaks after unsubscribing

[CJPrez]

The Client subscribe method has an bug in how it internally stores the handles to registered subscribers. The code starts like this:

 subscribe(fn: (state: ClientState<G>) => void) {
    const id = Object.keys(this.subscribers).length;
    this.subscribers[id] = fn;
    ...

It is using the count of subscribes as a property index to register a new subscriber. That will then break future subscribers if anyone but the most recent subscriber unsubscribes.

As an example scenario:

1. Subscriber A subscribes to the client and gets assigned ID 0
2. Subscriber B subscribes to the client and gets assigned ID 1
3. Subscriber A unsubscribes. The count of subscribers is now 1
4. Subscriber C subscribes to the client and gets assigned ID 1... again. Subscriber B no longer gets notifications.
5. Subscriber D subscribes to the client and gets assigned ID 1... again... again. Subscriber C no longer gets notifications.
6. etc...

The client implementation could keep the public facing API it has but internally use a CustomEvent object to store the subscribed handlers to avoid managing the subscriptions itself.