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/settings/defaults.py

@@ -576,9 +576,6 @@ ANSIBLE_INVENTORY_UNPARSED_FAILED = True
 # Additional environment variables to be passed to the ansible subprocesses
 AWX_TASK_ENV = {}
 
-# Flag to enable/disable updating hosts M2M when saving job events.
-CAPTURE_JOB_EVENT_HOSTS = False
-
 # Rebuild Host Smart Inventory memberships.
 AWX_REBUILD_SMART_MEMBERSHIP = False