Upload and download BitTorrent v1 file

[marcinczenko]

A more detailed tracking of this issue can be found in codex-storage/nim-codex#1137.

Initial implementation of the proposed protocol extension to support BitTorrent content.

Inputs:

• info BitTorrent v1 dictionary for a single file content,
• the corresponding info hash
• the corresponding content

Normally the user will just use the info hash to locate the content, but because in the end we will be integrating with a BitTorrent client, every time the user downloads BitTorrent content that the user also wants to seed to Codex network, we will use the very same info dictionary that was used by BitTorrent protocol, so that we have exactly the same matching triple: (content, info, info_hash).

The high level design is illustrated here.

Based on the input we will be storing something called a BitTorrentManifest, e.g.:

type
  BitTorrentPiece* = MultiHash
  BitTorrentInfo* = ref object
    length: uint64
    name: ?string
    pieceLength: uint32
    pieces: seq[BitTorrentPiece]

  BitTorrentManifest* = ref object
    info: BitTorrentInfo
    codexManifestCid: Cid

And we will make sure that the nodes having the relevant BitTorrent content advertise their SPRs under info hash in DHT.

Then when downloading, we:

1. fetch the BitTorrentManifest, from which we get the info dictionary, which we can use to compute info hash locally and check that it aligned with the request info hash.
2. form BitTorrentManifest we also get original codexManifestCid which will allows us to fetch the relevant blocks, and while getting them, we will be computing the corresponding hashes for the pieces, which we can validate using the data from the info dictionary that we preserved in the BitTorrentManifest, we can check that the reconstructed pieces match the original BitTorrent content.