This patch switches callbacks over to the payload object style events
[1] for PRECOMMIT_UPDATE based notifications. To do so a DBEventPayload
object is used with the publish() method to pass along the related data.
In addition a few UTs are updated to work with the changes. Finally
a few shims are put into place to allow PRECOMMIT_UPDATE based events to
use payloads while still supporting the existing kwarg style events.
NeutronLibImpact
[1] https://docs.openstack.org/neutron-lib/latest/contributor/callbacks.html#event-payloads
Change-Id: Ie6d27df01cd7b87894efc80946d41eb1ebe25bef
537bfb9a1c