Don't start profiler timer until disk is spun up

[lrettig]

I noticed that when I run the profiler on a dedicated disk that's in deep sleep, the first read is extra slow, but a subsequent read is faster:

> profiler -t 0 -n 144 --data-file /mnt/smesher-02/post/7c8cef2b/postdata_0.bin
{
  "time_s": 12.901740377,
  "speed_gib_s": 0.07750892288785359
}
> profiler -t 0 -n 144 --data-file /mnt/smesher-02/post/7c8cef2b/postdata_0.bin
{
  "time_s": 11.053322122,
  "speed_gib_s": 0.18094107616924474
}
> profiler -t 0 -n 144 --data-file /mnt/smesher-02/post/7c8cef2b/postdata_1.bin
{
  "time_s": 10.225100351,
  "speed_gib_s": 0.1955971023604089
}

(Changing nonces and threads here has no impact.)

Profiler should be smart enough to handle this case. It's probably as easy as having it perform one tiny "dummy read" before the timer starts, to make sure the disk is awake.