cancel non critical requests when setting the first time critical one. cleanup streaming code a bit with more comments

git-svn-id: http://libtorrent.googlecode.com/svn@9983 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
merged streaming changes from libtorrent_aio into trunk

git-svn-id: http://libtorrent.googlecode.com/svn@9984 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
fix incorrect bind retry count default setting

git-svn-id: http://libtorrent.googlecode.com/svn@9985 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda