parent: fire disconnect upon receiving DEL_ROUTE for context

This lets context.shutdown(wait=True) succeed in the master or any
parent.

mitogen/parent.py

@@ -619,6 +619,9 @@ class RouteMonitor(object):
         stream.routes.discard(target_id)
         self.router.del_route(target_id)
         self.propagate(mitogen.core.DEL_ROUTE, target_id)
+        context = self.router.context_by_id(target_id, create=False)
+        if context:
+            mitogen.core.fire(context, 'disconnect')
 
 
 class Router(mitogen.core.Router):