There is a potential window of opportunity where races can happen in the
API on consistency group related actions, this patch removes those
windows of opportunity using compare-and-swap for DB updates.
Races have been removed in following actions:
- create
- delete
- update
- create_cgsnapshot
- delete_cgsnapshot
Specs: https://review.openstack.org/232599/
Implements: blueprint cinder-volume-active-active-support
Change-Id: I67aec4cd8bcf2f7e09473a8d296aa383fe85ad23