Possible issues with on_watcher_queue_updated callback #45

When looking at this code with @codebytere for us to make a PR to propose upstream, we found a couple of things that we weren't sure about and would like another opinion on:

1. Most of the 'queue updated' calls work like this:

void uv__io_start(uv_loop_t* loop, uv__io_t* w, unsigned int events) {
  ...
  else if (QUEUE_EMPTY(&w->watcher_queue)) {
    QUEUE_INSERT_TAIL(&loop->watcher_queue, &w->watcher_queue);
    if (loop->on_watcher_queue_updated)
      loop->on_watcher_queue_updated(loop);
  }

But through the call chain of uv_loop_init() -> uv_async_init() -> uv__async_start() -> uv__io_start(), any loop already has an async watcher on it as soon as it's initialized. So if I'm reading this right, the callback will never be triggered for the async watcher because we'll never have an empty queue and a non-NULLon_queue_watcher_updated

2. One of the callback triggers doesn't look like the others. uv__io_feed() pumps it unconditionally, regardless of whether the loop->watcher_queue changes:

void uv__io_feed(uv_loop_t* loop, uv__io_t* w) {
  if (QUEUE_EMPTY(&w->pending_queue))
    QUEUE_INSERT_TAIL(&loop->pending_queue, &w->pending_queue);
  if (loop->on_watcher_queue_updated)
    loop->on_watcher_queue_updated(loop);
}

Are these intentional? If so, could you walk us through it so that we can better understand this code?