Delete snapshots - handle snapshot lookup and make it concurrent

[JeffreyDevloo]

• Deleting a large number of snapshots sequentially is very slow. We can optimize it by working storagedriver based and launch multiple concurrent tasks this way
• vdisk.snapshot_ids can throw errors if the volumedriver does not respond. This path is currently not caught and causes the whole task to bail out

[JeffreyDevloo]

Will be implemented by #2298

[JeffreyDevloo]

Framework concurrency added by #2298.
This implementation still iterates on the snapshots, causing a snapshot.xml update each time.

The volumedriver implemented a delete_snapshots call which can be used instead: https://github.com/openvstorage/volumedriver-ee/issues/207