Stop MSG2_STREAM_REGISTER broadcasting to limit vehicle number

[MxtAppz]

Instead of the current approach of kicking players who exceed the number of concurrent actors, it would be more elegant to show a warning to the client and don't broadcast to other clients, unless another actor is unregistered and releases a slot.

A more complex solution would be to handle the vehicle limit on the client side. Clients could (either explicitly or requiring manual action) let users decide whether to render an actor. Currently, they can remove others' actors for themselves only after they've been rendered, which could already had caused performance issues.