cache: move open to thread pool

[Ka-Hing Cheung]

This has been running in our test colo for the past week with no ill effects.

On Wed, Jan 23, 2019 at 4:39 AM Maxim Konovalov <maxim at nginx.com> wrote:
>
> Hi Ka-Hing,
>
> Roman told me that the delta is because of your changes.
>
> Thanks for your time on that.  Waiting for your testing results.
>
> Maxim
>
> On 22/01/2019 22:34, Ka-Hing Cheung via nginx-devel wrote:
> > I spoke too soon, just realized our test colo is running the patches
> > without aio_open on. Flipping that switch now.
> >
> > Also, including the patch that we applied on top in addition to the
> > massaging we did to resolve conflicts. I haven't dug too deep to see
> > if stock nginx also requires similar changes or they are only
> > necessary because of our other nginx changes:
> >
> [...]
>
> --
> Maxim Konovalov