Fix partially done downloads and choke groups

[chros73]

Depends on:

• Fix partially done downloads and choke groups libtorrent#147 (Fix partially done downloads and choke groups)

For more info:

• Bug in handling choke groups #564 (Bug in handling choke groups)
• rtorrent 0.9.3/0.13.3: Issues with "partially" downloaded torrent. Never finishes, completion percentage. #144 (comment) (rtorrent 0.9.3/0.13.3: Issues with "partially" downloaded torrent. Never finishes, completion percentage)

Also closes:

• stuck upload slots #348 (stuck upload slots)

[chros73]

I modified the pull requests:

• fix casting error in set_selected_size_bytes function in libtorrent
• save it into session and reload it during restart (similarly to chunks_wanted) (libtorrent is also modified)
  • required by stopped torrents (update_priorities() method isn't triggered for them upon start)
• remove d.set_selected_size_bytes command: it's not useful and not needed

I also updated the description with the changes.

[Jolly-Pirate]

Why hasn't this been merged in the past? It's a very much needed patch.