This patch switches BEFORE_DELETE callback events for AGENT resources
over to the payload style args use a DBEventPayload object. In addition,
the _delete_mac_associated_with_agent is now refactored into it's own
method to allow the context parameter to be parsed by
retry_if_session_inactive.
NeutronLibImpact
Change-Id: I0af951f4346f9290c2eba6aad17f772fbf27bb2f