on Linux, use CLOCK_MONOTONIC for clock_gettime()

On Linux systems which are being driven by an external time source (NTP or PTP), it is possible that time appears to slew in reverse under `CLOCK_REALTIME`. Since the timer function is used to time durations of events (calls to `select()`), it is better to use `CLOCK_MONOTONIC`, which isn't subject to slewing.

src/impl/unix.cc

@@ -91,7 +91,7 @@ MillisecondTimer::timespec_now ()
   time.tv_sec = mts.tv_sec;
   time.tv_nsec = mts.tv_nsec;
 # else
-  clock_gettime(CLOCK_REALTIME, &time);
+  clock_gettime(CLOCK_MONOTONIC, &time);
 # endif
   return time;
 }