reactor: Use the system monotonic clock instead of the normal system clock

The normal system clock can have sudden jumps if the system clock is
changed.  Use the system monotonic clock to avoid these sudden changes
in time.

It appears the Raspbian OS (which is used by OctoPi) is setup to
update the system clock upon network connectivity.  This could cause
sudden system clock changes which could lead to Klippy processing
errors.  Using the monotonic clock eliminates these issues.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/pyhelper.h

@@ -1,7 +1,7 @@
 #ifndef PYHELPER_H
 #define PYHELPER_H
 
-double get_time(void);
+double get_monotonic(void);
 struct timespec fill_time(double time);
 void set_python_logging_callback(void (*func)(const char *));
 void errorf(const char *fmt, ...) __attribute__ ((format (printf, 1, 2)));