nghttp: Add --header-table-size option
Enfoce flow control error strictly

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,
nghttpx: Remove flow control error detection code

Now flow control error detection is handled by the library
nghttpx: Fix stream hang with request body

The end of request stream is not detected correct place.
Also Downstream::end_upload_data() is not called.
Add new API to return effective received data length and local window size
nghttpx: Remove recv data length calculation in HTTP/2 code
Add new API to return effective recv data/win size for connection

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