Stop receive thread on unload.

Author: Mike Stegeman

gateway_addon/adapter.py

@@ -157,3 +157,4 @@ def cancel_remove_thing(self, device_id):
     def unload(self):
         """Perform any necessary cleanup before adapter is shut down."""
         print('Adapter:', self.name, 'unloaded')
+        self.manager_proxy.running = False