Make load average sampling asynchronous to hardclock ticks. This improves
measurement of load caused by time-related events still using hardclock.
For example, without this change dummynet, scheduling events each hardclock
tick, was always miscounted as load of 1.

There is still aliasing with events delayed by the new precision mechanism,
but it probably can't be avoided without moving this sampling from using
callout to some lower-level code or handling it in some other special way.

Reviewed by:	davide
Approved by:	re (marius)