simplestreams by-hash storage

[Description]
Currently, simplestreams is is able to mirror a source to a local filesystem by paths listsed.
For example:
 sstream-mirror --keep --max=1 http://cloud-images.ubuntu.com/releases/ out.d

That will mirror content from the download stream into out.d/ with paths like server/releases/saucy/release-20140129/ubuntu-13.10-server-cloudimg-amd64.tar.gz .
http://cloud-images.ubuntu.com/releases/streams/v1/com.ubuntu.cloud:released:download.json

That is safe (per definition of simplestreams format) and external paths are checked for safety.

The issue is that if multiple paths exist to the same sha256 sum, then we'll download those files mutiple times. The files could be listed on completely separate sites or inside of different content on the same site.

This is wasteful if downloads are hundreds of megabytes.

[Changes]

The goal is to just change simplestreams to be able to store "by-hash", and create hardlink or symlink trees from the paths listed in the source.

The format used for the by-hash storage would be just like git's .
 <storage>[A-Z][A-Z]/....