[SAP] Improve parallel creation from snapshot/volume

[leust]

The source volume/snapshot was locked while the new volume was creating, preventing other volumes from being created in parallel from the same source. This was to prevent the source from being deleted while the volume is being created from it.

We move that lock from the manager upper to the API layer until the volume reaches the 'creating' state, then the lock is released.

The delete_snapshot / delete_volume will check the DB if there are volumes creating from it before proceeding with the deletion.

The caveat is that, if a volume is stuck in 'creating', it will still prevent the source from being deleted.

Change-Id: I8be26242474a2b4eba676978b533f8cad40e292b

[hemna]

So the current API is that if the lock is in place, that the delete volume just waits to acquire the lock and doesn't report an error in the API. We should try and maintain that, so there is no change to the API response.

[leust]

In this case I think is easier to introduce a new API microversion for supporting parallelism in cloning.

I don't see a good solution for how to wait for all clone operations to finish, apart from looking into the database, which might lead to inconsistencies.