Backup/recover torrent persistent data into each individual .fastresume file. This should mitigate the problem of users losing their torrents' settings/savepath/label after qbt wasn't shutdown cleanly.
Optimize torrentperistentdata parameters and add utility funtions for previous commit.
Don't save resume data when torrent has an error. Prevents removing possibly useful .fastresume file and writing garbage.
Fix compilation with libtorrent 0.15.x.
Really fix compilation with 0.15.x.