When using WSGI module, the multiprocess event is not shared between
the WSGI workers. This new implementation retrieves the WSGI start time
to define the OVN hash ring register creation time. That will be used
to filter out the stale registers.
Depends-On: https://review.opendev.org/c/openstack/devstack/+/936669
Closes-Bug: #2083570
Change-Id: Id9f851f33c2cb3d2c2759a3c66adf2599a3122fe