optimize the callback receiver to buffer writes on high throughput

additionaly, optimize away several per-event host lookups and
changed/failed propagation lookups

we've always performed these (fairly expensive) queries *on every event
save* - if you're processing tens of thousands of events in short
bursts, this is way too slow

this commit also introduces a new command for profiling the insertion
rate of events, `awx-manage callback_stats`

see: https://github.com/ansible/awx/issues/5514

awx/main/dispatch/pool.py

@@ -277,7 +277,7 @@ class WorkerPool(object):
                 logger.warn("could not write to queue %s" % preferred_queue)
                 logger.warn("detail: {}".format(tb))
             write_attempt_order.append(preferred_queue)
-        logger.warn("could not write payload to any queue, attempted order: {}".format(write_attempt_order))
+        logger.error("could not write payload to any queue, attempted order: {}".format(write_attempt_order))
         return None
 
     def stop(self, signum):