Reduce the number of torrent_handle::torrent_file() calls in QTorrentHandle::prioritize_files()

It was reported (#2161) that enabling/disabling a downloading of a file
is considerably slow on libtorrent 1.0.3, but not on 0.16.x. The problem
is that a function torrent_file() in libttorrent 1.0.3 does a deep copy
of torrent_info, while get_torrent_info() in libtorrent 0.16.x only
returns a reference.

This is how is was on libtorrent 1.0.3 before this commit:
![prio-1](https://cloud.githubusercontent.com/assets/286877/5055024/0ea1e86a-6c6e-11e4-94dd-4fb5f7158cab.png)
![prio-2](https://cloud.githubusercontent.com/assets/286877/5055025/0ea6f9b8-6c6e-11e4-991f-cc4c23912cdf.png)

This is how it is now:
![prio-3](https://cloud.githubusercontent.com/assets/286877/5055028/1d57fffc-6c6e-11e4-85c8-416974fd026e.png)

Note, absolute time doesn't matter. Look only on ratio of time spend by different functions.