Relax the timing constrains for disk IO to accommodate rotating disks;
a "measured IO" might be the result of a small number of physical IOs,
allow for enough time for a small number of disk rotations (this isn't
perfect but seems to be about right in testing under load).
Change-Id: Ifb067a2218528e5918d2f4b2ba169b6e739084e0
d7808468fc