Summary: Before this, we directly mounted the the rawfile on the mountpoint. In this revision the `STAGE_UNSTAGE_VOLUME` capability is implemented, meaning that the volume is first mounted to a staging path, and then `bind`-mounted to the actual path. This way we can free up loopback devices when they are not needed.
Test Plan:
- Create a pvc, and use it inside a pod
- Run `losetup -l` on the node running the pod, and assert the creation of a loop device
- Delete the pod, but not the pvc
- Run `losetup -l` on the same node, and assert the removal of the loop device
Reviewers: h.marvi, bghadiri
Differential Revision: https://phab.hamravesh.ir/D806
