Keep notifier core API consistent

Issue 1:
The callbacks appliance in `unsubscribe` needs to be removed in order to keep consistent the notifiers core API. For instance, in vert.x notifier the callback in `unsubscribe` must be the same callback used in `subscribe` and not a callback which informs that the `unsubscribe` command was sent to the server.

Issue 2:
I have changed the implementation for AGJS-128 because if `unsubscribe` is called by passing the `client.getChannels()` channels which already have the channel id then the code which finds the `id` in `unsubscribe is redundant. So it is better to check if the each channel has an id and if it doesn't then to find it.