rb://566
approved by: Sunny

When using native aio on linux each IO helper thread should be able to
handle upto 256 IO requests. The number 256 is the same which is used
for simulated aio as well. In case of windows where we also use native
aio this limit is 32 because of OS constraints. It seems that we are
using the limit of 32 for all the platforms where we are using native
aio. The fix is to use 256 on all platforms except windows (when native
aio is enabled on windows)