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