use synchronized decorator from neutron-lib
use synchronized decorator from neutron-lib

neutron-lib contains the synchronized lock decorator and neutron is
removing it's implementation I729da348e340509f2d09f8a6436716e2398f1583

This patch uses the decorator from neutron-lib rather than neutron.

Change-Id: I25c5b86a586c672e4505d895f5d2b7d7eb276910

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