MEDIUM: clock: use the monotonic clock for idle time calculation

MEDIUM: clock: use the monotonic clock for idle time calculation

By just keeping a copy of the last known value before entering
polling, we can apply the same algorithm as we're currently using,
except that it's now applied to the monotonic clock instead of the
wall clock, when it's detected that it's ticking. This improves
idle time calculation accuracy by making it independent on the
wall clock.