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/settings/defaults.py

@@ -197,6 +197,14 @@ LOCAL_STDOUT_EXPIRE_TIME = 2592000
 # events into the database
 JOB_EVENT_WORKERS = 4
 
+# The number of seconds (must be an integer) to buffer callback receiver bulk
+# writes in memory before flushing via JobEvent.objects.bulk_create()
+JOB_EVENT_BUFFER_SECONDS = 1
+
+# The interval at which callback receiver statistics should be
+# recorded
+JOB_EVENT_STATISTICS_INTERVAL = 5
+
 # The maximum size of the job event worker queue before requests are blocked
 JOB_EVENT_MAX_QUEUE_SIZE = 10000