(trunk libT) #1748: possible fix for the kqueue corruption errors by consolidating...

(trunk libT) #1748: possible fix for the kqueue corruption errors by consolidating the three per-torrent libevent timers into three session-wide timers.  Since most people reporting this error have lots of torrents loaded, consider a hypothetical example: if you had 500 torrents, this patch will reduce 1,500 libevent timers down to just three timers.  On top of that, those three have simpler life cycles too...