unix: try to write immediately in uv_udp_send
test: fix watcher_cross_stop

It's possible that recv_cb_called is bigger than the number of sockets,
because if all sends succeed the recv callback is called twice: once
with the actual data, and another time with 0.
windows: count queued bytes even if request completed immediately

The send_queue_size/count needs to maintain its value until right before
the callback is called.