merged changes from libtorrent_aio

git-svn-id: http://libtorrent.googlecode.com/svn@10046 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
separate session from peer_connections a bit more

git-svn-id: http://libtorrent.googlecode.com/svn@10047 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
remove pch.hpp

git-svn-id: http://libtorrent.googlecode.com/svn@10048 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
initialize pad files to priority 0 by default

git-svn-id: http://libtorrent.googlecode.com/svn@10049 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda