add waiting_for_response indicator on User

.spawn_pending used for the *whole* window, from request to responsive (added to proxy)
.waiting_for_response is just used for the window between Spawner.start returning (process started, http endpoint known) and http endpoint becoming responsive

.waiting_for_response will never be True while .spawn_pending is False

jupyterhub/user.py

@@ -99,6 +99,7 @@ class User(HasTraits):
     spawner = None
     spawn_pending = False
     stop_pending = False
+    waiting_for_response = False
     
     @property
     def authenticator(self):
@@ -258,6 +259,7 @@ class User(HasTraits):
         self.state = spawner.get_state()
         self.last_activity = datetime.utcnow()
         db.commit()
+        self.waiting_for_response = True
         try:
             yield self.server.wait_up(http=True, timeout=spawner.http_timeout)
         except Exception as e:
@@ -284,7 +286,9 @@ class User(HasTraits):
                 ), exc_info=True)
             # raise original TimeoutError
             raise e
-        self.spawn_pending = False
+        finally:
+            self.waiting_for_response = False
+            self.spawn_pending = False
         return self
 
     @gen.coroutine