minor disk thread assert and tweak

git-svn-id: http://libtorrent.googlecode.com/svn@9966 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
merged changes from trunk

git-svn-id: http://libtorrent.googlecode.com/svn@9967 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
improve queuing logic of inactive torrents (dont_count_slow_torrents)

git-svn-id: http://libtorrent.googlecode.com/svn@9968 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
downloaders only consider download threshold and seeds only consider upload thresholds, for determining whether a torrent is considered inactive or not

git-svn-id: http://libtorrent.googlecode.com/svn@9969 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda