stop using common db mixin
stop using common db mixin

All of the methods of common db mixin are available via neutron-lib
and the mixin will be removed before long.
This patch switches the code over to use neutron-lib's APIs rather
than those of the mixin as well as top using the mixin as a parent
class.

Change-Id: Ibf79fa3c7077aea023d882dd6bd562261d093d8d

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl