In order to keep the performance we made it Internal Mutability and use a `Mutex` per Stream, till `Stream.send` is no longer `&mut self`.
The old solution didn't rely on this, but needed multiple Components instead which zest didn't liked
- fix wording in error msg
- find better name for structs
- unify errors and cleanup code with `(|| {foo?; Some(())})()` pattern
- fix the negative PlayersOnline, it was caused by having a gracefull shutdown AND a timeout error. we now unregister the client when he issues TERMINATE
