[Feature request] Implement integrity check

[matveevandrey]

It would be cool to have an ability to perform integrity check on stored images

To reduce memory usage checksum should be calculated for stream instead of file thus we have only double read: compare( read(base_image +diffs) vs read(current_rbd_snapshot))

Yes this function will be resource consuming but if it applies once per week/month I think it would be enough.

However in my understanding this feature could only be implemented after adding:
#36

[lephisto]

Done here: lephisto@0886ef1