Bittorrent v2 metadata fetching

[the8472]

• juggle multiple infohashes. hybrid torrents and the associated magnets can have two. we might only know the 20byte "identifier" prefix of a v2 hash if we captured it from the DHT
• fetch piece layers root dictionary entry via hash request message
• detect v2 content in a v1 torrent and try to upgrade

Work will be preliminary until there is some other implementation to test against.

[mweinelt]

There seems to be some bittorrent v2 support in transmission now, wondering if that helps.

https://github.com/transmission/transmission/releases/tag/4.0.0