It is effectively dead code because we should have already submit
WINDOW_UPDATE before this case happens.

Using this feature, connection level flow control is now enabled
in nghttpx.

The end of request stream is not detected correct place.
Also Downstream::end_upload_data() is not called.

Now flow control error detection is handled by the library

Now we have SETTINGS synchronization, flow control error can be
detected strictly. If DATA frame is received with length > 0 and
current received window size is equal to or larger than local
window size (latter happens when we shirnk window size), it is
subject to FLOW_CONTROL_ERROR,

SETTINGS timeout is 10 seconds for now.