net.box: avoid unnecessary syscalls when socket is ready most of the time

Don't run event loop if socket is ready to write. *Always* write
optimistically. With fiber.channel(), channel push  wakes up the waiter
without in the same event loop iteration. This means that whenever we have
data on an incoming connection, we can push messages to outgoing connections
in the same iteration, without epoll_ctl syscalls or unnecessary
epoll_wait.
With 1 incoming and 5 outgoing connections, for example, this saves
~10 syscalls per each incoming request.