more steps towards isolating peer_connections from the session object. unchoking now happens via the torrent object. hopefully in the future, the session can determine which peers to unchoke without having direct access to them (i.e. with a mutex)

git-svn-id: svn://svn.code.sf.net/p/libtorrent/code/trunk@10319 a83610d8-ad2a-0410-a6ab-fc0612d85776