EVENT_BASE_FLAG_PRECISE_TIMER indicates we want fine timer precision

There are a bunch of backends that can give us a reasonably good
monotonic timer quickly, or a very precise monotonic timer less
quickly.  For example, Linux has CLOCK_MONOTONIC_COARSE (1msec
precision), which is over twice as fast as CLOCK_MONOTONIC.  Since
epoll only lets you wait with 1msec precision,
CLOCK_MONOTONIC_COARSE is a clear win.

On Windows, you've got GetTickCount{,64}() which is fast, and
QueryPerformanceCounter, which is precise but slow.

Note that even in the cases where the underlying timer claims to
have nanosecond resolution, we're still not exposing that via
Libevent.

Note also that "Precision" isn't the same as "Resolution" or
"Accuracy".  A timer's precision is the smallest change that the
clock will register; a timer's resolution is the fineness of its
underlying representation; a timer's accuracy is how much it drifts
with respect to "Real Time", whatever that means.  (Terms and
definitions from PEP 418.)

include/event2/event.h

@@ -481,7 +481,14 @@ enum event_base_config_flag {
 	    This flag has no effect if you wind up using a backend other than
 	    epoll.
 	 */
-	EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST = 0x10
+	EVENT_BASE_FLAG_EPOLL_USE_CHANGELIST = 0x10,
+
+	/** Ordinarily, Libevent implements its time and timeout code using
+	    the fastest monotonic timer that we have.  If this flag is set,
+	    however, we use less efficient more precise timer, assuming one is
+	    present.
+	 */
+	EVENT_BASE_FLAG_PRECISE_TIMER = 0x20
 };
 
 /**