nanosecond: reconcile terse output with nanosecond timing for latencies

nanosecond: reconcile terse output with nanosecond timing for latencies

The terse output format is unchanged and latencies are still reported
as usec. Completion latencies are tracked with nsec precision in io_u_lat_n.
Put these latencies in the first microsecond latency bucket which is where they
would have fallen before the change to nsec timing.