perfcounters: fl -> time, use u64 nsec instead of double

perfcounters: fl -> time, use u64 nsec instead of double

(Almost) all current float users are actually time values, so switch to
a utime_t-based interface and internally using nsec in a u64.  This avoids
using floating point in librbd, which is problematic for windows VMs that
leave the FPU in an unfriendly state.

There are two non-time users in the mds and osd that log the CPU load.
Just multiply those values by 100 and report as ints instead.

Fixes: #3521
Signed-off-by: Sage Weil <sage@inktank.com>