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).