The call to put IO threads to sleep was most probably meant for Windows
only as the comment in buf0rea.c suggests. However it was enabled on
all platforms. This patch restricts the sleep call to windows. This
approach of not putting threads to sleep makes even more sense because
now we have multiple threads working in the background and it probably
is not a good idea to put all of them to sleep because a user thread
wants to post a batch for readahead.

Approved by: Marko