workflows: new stage for halted

* Adds 'WAITING' stage for halted workflows with
  no action added to them.
I wonder if we shall make this one `4` instead of the same. Because it is then consistent. It shouldn't be a big deal if existing halted objects is not waiting. 

Then instead we check in `client.py` if an action is attached or not and set the right `ObjectVersion`. This way, no logic is needed in templates.