RFC: Allow cosockets to be accessed across requests

I'm working on trying remove the restriction added many many years ago (https://github.com/openresty/lua-nginx-module/commit/414db1a80b489cca4fbab96fddc018ccd61dab78). As far as I can tell, there's no intrinsic reason why we can't make this work, it just requires careful analysis across any yield points.

Any advice for me while I work on this?