Raise libevent dependency to 2.0.10-stable or newer

Only some very ancient distributions don't ship with Libevent 2 anymore,
even the oldest supported Ubuntu LTS version has it. This allows us to
get rid of a lot of compat code.

src/test/test_workqueue.c

@@ -12,11 +12,7 @@
 #include "compat_libevent.h"
 
 #include <stdio.h>
-#ifdef HAVE_EVENT2_EVENT_H
 #include <event2/event.h>
-#else
-#include <event.h>
-#endif
 
 #define MAX_INFLIGHT (1<<16)