Plan to integrate with vagga

[tailhook]

Okay, so now we can upload a single image from client to server. It can't be used for p2p purposes yet, but we can use it to upload images for the vagga cache.

1. Check if metadata storage is enough: it records only upload date for now.
2. Implement cleanup settings and algorithm (last N uploaded images) Image cleanup #2
3. Implement ciruela download command to actually download blocks
4. Wrap client into a library, embed into vagga
5. Implement hardlinking on server-side

Eventually, we may need some access log for image tracking, but hopefully not immediately.

/cc @anti-social