cleaned up redundant bytes and failed bytes counters. moved some into the stats_counters

git-svn-id: http://svn.code.sf.net/p/libtorrent/code/trunk@10165 a83610d8-ad2a-0410-a6ab-fc0612d85776
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://svn.code.sf.net/p/libtorrent/code/trunk@10166 a83610d8-ad2a-0410-a6ab-fc0612d85776