Return success immediately if a file exists

We need to design in a sane system for detecting if a file is already on disk. Furth we need to make sure that we do not download the same file twice at the same time. This will likely require that the user requests this feature and provides a checksum.

This will help the following use case:

nova-compute requests a file. Nova then dies but staccato does not. Staccato continues to download the file. Nova compute then comes back up and a user request the same file transfer. This should be detected so as to not happen twice.