Ensure security on Quota measurement

It's currently using cmd = 'du -c %s | tail -n 1' % start_path

We should use something from Python or sanitize start_path.

cmd vs python

According to this question doing it in Python is very slow if compared to du. We should implement some good sanitizing on path instead.

live measurements

Another requirement for this feature is that a huge blob can't be stored if it's bigger than unused quota. This needs to be checked during transfer on server site.