Add x-i2p-torrentlocation support
If a user of apt-transport-i2phttp has downloaded from a repository which enables the x-i2p-torrentlocation header, or which publishes an x-i2p-torrentlocation meta tag on it's index.html, then apt-transport-i2phttp should use the torrent location first, but verify the package with the hash from the http source(and if it is incorrect, download the new version of the torrent).