mgr/volumes: Introduce v2 subvolumes

mgr/volumes: Introduce v2 subvolumes

Implements subvolume v2 version.

Following support is added,
- Ability to retain snapshots on subvolume deletion
- Modify directory where snapshot is created to the subvolume
- "features" supported to subvolume info output, specifically ability
for a subvolume to retain snashots
- Current state of subvolume in info output
- Auto upgrade to v2 from eligible v1 subvolumes
- Adjust other functions as needed to support the changes

Signed-off-by: Shyamsundar Ranganathan <srangana@redhat.com>