[core] Implement io_uring

[ammarfaizi2]

I am going to replace epoll with io_uring, however io_uring syscall is only available for Linux 5.1+, hence I will not drop the epoll support, I will just make a dispatcher to detect kernel version at runtime.

IOW, we can say like this:

if (linux_version >= 5.1) {
   // use io_uring
} else {
   // fallback to epoll
}