[Optimise] Use ThredGroup to manage periodic tasks
[Optimise] Use ThredGroup to manage periodic tasks

Instead of manually managing periodic tasks, use
oslo.service's threadgroup.
[1]:https://docs.openstack.org/oslo.service/latest/reference/threadgroup.html
[2]:https://review.opendev.org/c/openstack/cinder/+/558416

Change-Id: I43a55f0ffff3a0dc6b9fc757869e1e3c3240716f

openstack/manila,openstack/manila