Use io_prio as mainloop prio for async-emulation (#579449)

Use io_prio as mainloop prio for async-emulation (#579449)

I'm not sure why we used the elaborate formula to convert the io-priority
to the priority of the mainloop idle when emulating async i/o with idles.
However, it causes the default io priority to be less than the normal
idle prio, so the i/o won't be scheduled if there is an idle outstanding.

There is really no great mapping to use here, doing blocking i/o in an
idle of any prio is generally bad and apps doing a lot of async i/o should
initialize threads. However, if we use the io-priority directly we at least
avoid the starvation problem above and make things easier to understand.