make performance counters able to blend in values, reducing a peer_connection -> session dependency on a sliding average state (this can be used for disk stats too)

git-svn-id: http://libtorrent.googlecode.com/svn@10166 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
cleanup and make session_stats a bit more efficient

git-svn-id: http://libtorrent.googlecode.com/svn@10167 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda