This makes ReadAt for non cached files wait a short time (up to 5mS)
if it gets an out of order read (which would normally cause a seek and
which take a long time) to see if the gap will be filled with an in
order read.
This makes mount2 based on go-fuse work more efficiently and enables
async reading in normal mount.
A similar change was done for WriteAt in af030f74f5