Convert group_run_stats to use bytes instead of KiB/KB

Convert group_run_stats to use bytes instead of KiB/KB

In group_run_stats, convert these fields:
* io_kb (bytes transferred)
* agg (aggregate average bandwidth)
* min_bw
* max_bw

from tracking either KiB or KB depending on kb_base to simply tracking bytes.
Rename io_kb to iobytes to match this new meaning.

This will simplify computing quantities and bandwidth rates with both IEC
binary and SI decimal unit prefixes (e.g., GiB/s and GB/s) in future patches.

Signed-off-by: Jens Axboe <axboe@fb.com>