Investigate use of batch notification listener

[Steve McLellan]

Ceilometer switched to using oslo.messaging's BatchNotificationListener in https://review.openstack.org/#/c/250814/, presumably for performance reasons. Since searchlight will be subject to the same performance problems ceilometer has experienced with throughput, we should look into the reasons the change was made and consider implementing it also.

[Steve McLellan (sjmc7)]

On revisiting this while not in the throes of a release, I suspect we may not want this after all. The batched listener was developed to assist with a ceilometer performance problem; the solution was to insert events into the ceilo database in batches, which in turn led to receiving notifications in batches (http://permalink.gmane.org/gmane.comp.cloud.openstack.devel/12454).

Elasticsearch doesn't particularly suffer from this, and more importantly, our architecture doesn't allow it right now (even if we received notifications in batches, each plugin would still insert them one at a time).

I'm going to recommend closing this as WONT_FIX for now; if we experience performance issues and decide that this might help, it should be a blueprint of its own.