object-updater once mode ignores updater_workers

[clayg]

The "workers" option for the object_updater is implemented inline [1] instead of using the DaemonStrategy [2]

I'm not sure how easy it would be to convert it - but one effect of the custom implementation is that when using "once mode" there is no forking [3]

Ideally both the run_once and run_forever methods would take advantage of multiple workers when configured to do so.

1. https://github.com/openstack/swift/blob/master/swift/obj/updater.py#L159
2. https://github.com/openstack/swift/blob/master/swift/common/daemon.py#L225
3. https://github.com/openstack/swift/blob/master/swift/obj/updater.py#L190