feat: sequential download

[Donpuchipuchi]

Allows to optionally sequentially download a torrent - closes #597. Based off @rnhmjoj's patch ported to transmission v4.

A few caveats / open-questions :

• no GUI implementation yet
• no web-ui implementation yet
• defaults to off - should there also be a globally settable-option ?
• usage is a bit counter intuitive (more on this later)
• I'm not a c++ programmer !

The usage can be counter intuitive - e.g. when sequentially downloading the free tears of steel torrent, if I only enable the movie, it will download it sequentially, but it will end with the beginning of the file - effectively preventing streaming the file from the disk with a video-player.

Knowing this, an easy workaround here is to download everything but the movie first, and then the movie. But I can imagine this would not immediately obvious to most users ! I don't see an immediate solution for this - I just wanted to share along with this PR to get ideas.

Thank you !

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. See the FAQ for more information.