Storwize/SVC: Change volume copy task to async

Storwize/SVC: Change volume copy task to async

If Cinder crashes during a migration or retype (where data is moved and
the operation can take a long time), the storage ends up with multiple
copies of the same volume which requires storage admin intervention.

This patch maintain a list of pending operations which is backed up in
admin metadata, and a periodic task reviews the list and removes volume
copies whose copy operation completed. When Cinder comes up, check the
admin metadata and rebuild the list.

Change-Id: I6549712bb0083996faced89c2207a4c438ae953d
Closes-Bug: #1278035