Improve rwmix buffered split

Improve rwmix buffered split

For buffered IO, the rwmix split can often be grossly unfair
towards reads, because dirtying tons of memory is done much
faster than reading data. Improve the split for such workloads
by not only looking at time.

Note that it'll still be somewhat unfair, there's only so much
we can reliably do. But it's better.

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