Built-up Changes to the WGPS

[AljoschaMeyer]

Do not merge yet, more changes to come.

This starts with adding (generalised) closing of logical channels to the WGPS. As a side-effect, I had to change the encodings of all control messages except ControlAnnounceDropping and ControlApologise. Nothing major though, these are all straight-forward to adjust (and the encodings for the two new message types, ControlLimitReceiving and ControlLimitSending, are very straightforward to implement as well).