ExternalInputDeviceHub::remove_observer() hangs

[AlanGriffiths]

Repeatedly running wlcs tests leads to an intermittent hang as follows.

1. The main loop has stopped.
2. The base thread is waiting on server.stop()
3. The AsyncServer thead is waiting on wayland->stop() (in DisplayServer::run())
4. The Wayland thread is waiting on ExternalInputDeviceHub::remove_observer()
5. This is waiting on a condition_variable that should be notified by functor enqueued with enqueue_with_guaranteed_execution()

This seems to be a race in that enqueue_with_guaranteed_execution() has enqueued the action.I.e. it has seen "running" as true (and not run the action on the calling thread).