Watcher set changes during iteration

[TheJJ]

I'm seeing several failures in job processing:

Sep 12 15:23:16 cyberkischt env[4331]: [2018-09-12 15:23:16,957] exception in Job.run() openage.arch-clang [ecaf496cde4fc12333d383875783c367cddbbbb6]                                                               
Sep 12 22:18:47 cyberkischt env[4331]: Traceback (most recent call last):                                                                                                                                           
Sep 12 22:18:47 cyberkischt env[4331]:   File "/usr/lib64/python3.6/site-packages/kevin/job.py", line 350, in run                                                                                                   
Sep 12 22:18:47 cyberkischt env[4331]:     await control_handler.asend(data)                                                                                                                                        
Sep 12 22:18:47 cyberkischt env[4331]:   File "/usr/lib64/python3.6/site-packages/kevin/job.py", line 563, in control_handler                                                                                       
Sep 12 22:18:47 cyberkischt env[4331]:     await self.control_message(msg)                                                                                                                                          
Sep 12 22:18:47 cyberkischt env[4331]:   File "/usr/lib64/python3.6/site-packages/kevin/job.py", line 581, in control_message                                                                                       
Sep 12 22:18:47 cyberkischt env[4331]:     await self.set_step_state(msg["step"], msg["state"], msg["text"])                                                                                                        
Sep 12 22:18:47 cyberkischt env[4331]:   File "/usr/lib64/python3.6/site-packages/kevin/job.py", line 287, in set_step_state                                                                                        
Sep 12 22:18:47 cyberkischt env[4331]:     time=time))                                                                                                                                                              
Sep 12 22:18:47 cyberkischt env[4331]:   File "/usr/lib64/python3.6/site-packages/kevin/watchable.py", line 55, in send_update                                                                                      
Sep 12 22:18:47 cyberkischt env[4331]:     await watcher.on_update(update)                                                                                                                                          
Sep 12 22:18:47 cyberkischt env[4331]:   File "/usr/lib64/python3.6/site-packages/kevin/build.py", line 387, in on_update                                                                                           
Sep 12 22:18:47 cyberkischt env[4331]:     lambda subscriber: isinstance(subscriber, Job)                                                                                                                           
Sep 12 22:18:47 cyberkischt env[4331]:   File "/usr/lib64/python3.6/site-packages/kevin/watchable.py", line 51, in send_update                                                                                      
Sep 12 22:18:47 cyberkischt env[4331]:     for watcher in self.watchers:                                                                                                                                            
Sep 12 22:18:47 cyberkischt env[4331]: RuntimeError: Set changed size during iteration         

Pls investigate and fix.