Plan to integrate with vagga

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. [x] Implement cleanup settings and algorithm (last N uploaded images) #2
3. [ ] Implement ciruela download command to actually download blocks
4. [ ] Wrap client into a library, embed into vagga
5. [x] Implement hardlinking on server-side

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

/cc @anti-social