When GUI disconnects on trafficsim interface, with high traffic, simulator locks up

[Sean Condon]

Then the simulator is heavily loaded, and the GUI is connected, and the GUI subsequently disconnected,
the clean up function does not complete - is locked by something.

The GUI can reconnect again, but this does not solve the lock.

For example when GUI disconnects:

{"level":"INFO","ts":"2020-06-29T12:38:37.582Z","northbound/trafficsim":"northbound/trafficsim","caller":"trafficsim/trafficsim.go:145","msg":"Client has disconnected ListCells on cell-0xc001928910"}
{"level":"INFO","ts":"2020-06-29T12:38:37.582Z","northbound/trafficsim":"northbound/trafficsim","caller":"trafficsim/trafficsim.go:200","msg":"Client has disconnected ListUes on ue-0xc002260f80"}
{"level":"INFO","ts":"2020-06-29T12:38:37.582Z","northbound/trafficsim":"northbound/trafficsim","caller":"trafficsim/trafficsim.go:91","msg":"Client has disconnected ListRoutes on route-0xc001ad5f60"}

There should be an extra line in there after the ListUes like:

{"level":"INFO","ts":"2020-06-29T13:28:25.949Z","dispatcher":"dispatcher","caller":"dispatcher/dispatcher.go:76","msg":"Unregistered ue-0xc002260f80 from UE listeners"}

It may mean that it's unable to acquire the lock on the d.nbiUeListenersLock. This might be because it's locked in ListenRouteEvents