core: RestoreAllSnapshots partial cleanup

- Replace the class member 'targetSnapshot' with local field.
- Change the type of deleteDisksIds from List to Set, which is more
  appropriate (as it should contain every value at most once), and
  it improves the performance of the loop in #deleteOrphanedImages.
- Change the type of 'snapshotsToRemove' member from List to Set,
  instead of ensuring that it doesn't contain any value more than once
  when inserting to it.

Change-Id: I0557a832915e00877f9d1ee79e4cf2d429537396
Signed-off-by: Arik Hadas <ahadas@redhat.com>