remove multiprocessing.Queue usage from the callback receiver

instead, just have each worker connect directly to redis
this has a few benefits:

- it's simpler to explain and debug
- back pressure on the queue keeps messages around in redis (which is
  observable, and survives the restart of Python processes)
- it's likely notably more performant at high loads

awx/main/dispatch/control.py

@@ -25,8 +25,14 @@ class Control(object):
 
     def status(self, *args, **kwargs):
         r = redis.Redis.from_url(settings.BROKER_URL)
-        stats = r.get(f'awx_{self.service}_statistics') or b''
-        return stats.decode('utf-8')
+        if self.service == 'dispatcher':
+            stats = r.get(f'awx_{self.service}_statistics') or b''
+            return stats.decode('utf-8')
+        else:
+            workers = []
+            for key in r.keys('awx_callback_receiver_statistics_*'):
+                workers.append(r.get(key).decode('utf-8'))
+            return '\n'.join(workers)
 
     def running(self, *args, **kwargs):
         return self.control_with_reply('running', *args, **kwargs)