windows: can't call try_write from a write callback

[PeterJohnson]

In https://github.com/libuv/libuv/blob/v1.x/src/win/tcp.c#L1127, the write() user callback is called before the write_reqs_pending is decremented (line 1130). As uv__tcp_try_write() unconditionally returns EAGAIN when write_reqs_pending is non-zero (line 952), this means it's impossible to have a successful try_write() call from a write() callback, even though the system should be fine with that. I think simply moving the decrement to before the callback would fix this with no other ill effects?

[bnoordhuis]

That sounds about right. PR welcome; a test would be nice.