MINOR: clock: add now_cpu_time_fast() function

MINOR: clock: add now_cpu_time_fast() function

Same as now_cpu_time(), but for fast queries (less accurate)
Relies on now_cpu_time() and now_mono_time_fast() is used
as a cache expiration hint to prevent now_cpu_time() from being
called too often since it is known to be quite expensive.

Depends on commit "MINOR: clock: add now_mono_time_fast() function"