cleanup and make session_stats a bit more efficient

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@10167 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix docs generation for stats counters

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@10168 a83610d8-ad2a-0410-a6ab-fc0612d85776