Update Tor to use Libevent 2.0 APIs when available.
This patch adds a new compat_libevent.[ch] set of files, and moves our Libevent compatibility and utilitity functions there. We build them into a separate .a so that nothing else in src/commmon depends on Libevent (partially fixing bug 507). Also, do not use our own built-in evdns copy when we have Libevent 2.0, whose evdns is finally good enough (thus fixing Bug 920).