improve robustness of test_privacy and error messages of test_fast_extension

git-svn-id: http://libtorrent.googlecode.com/svn/trunk@10170 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
deprecate parts of cache_status and move those counters to session_stats counters. make the performance counters thread safe. fix some issues recently introduced to performance counters and the usage of them in client_test

git-svn-id: http://libtorrent.googlecode.com/svn/trunk@10171 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda