[BUG] subscribers may be removed/changed from a channel when updating priority

[Tieske]

in channel:setPriority the subscriber is looked up by its id. This is done by calling channel:getSubscriber. However getSubscriber will search the current channel, and all of its child/sub channels. So the subscriber being returned might actually be in a child/sub-channel.

When removing/adding the subscriber the channel:setPriority now removes/adds the subscriber from the current channel, even though it might have been found in a child/sub-channel.