Fix partially done downloads and choke groups

[chros73]

Depended by:

• Fix partially done downloads and choke groups rtorrent#575 (Fix partially done downloads and choke groups)

For more info:

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

Also closes:

• stuck upload slots rtorrent#348 (stuck upload slots)

[coveralls]

Coverage decreased (-0.03%) to 18.007% when pulling 5af9e51 on chros73:partially_done_and_choke_group_fix into c167c5a on rakshasa:master.