The nghttp2_submit_{request,response} family do not get this change.

We wrongly added the whole payload length even if we sent part of it.

It turns out that 0-bit prefix is wrong, and the author now
clearly stated that the intention is 8-bit prefix for 8+ fields.

Now shrinking local window size properly limits the amount of
WINDOW_UPDATE value so that shrinked window is honored.

Use NGHTTP2_ERR_INVALID_STREAM_STATE when trying to send DATA or
WINDOW_UPDATE to reserved stream.
Also a bit refactored nghttp2_session_predicate_window_update_send
and nghttp2_session_predicate_data_send

The NGHTTP2_ERR_PAUSE library error code is introduced to pause
the execution of nghttp2_session_mem_recv() when that error code
is returned from nghttp2_on_frame_recv_callback or
nghttp2_on_data_chunk_recv_callback. If this happens, the parameters
available for both callbacks are retained until the application
calls nghttp2_session_continue(). The application must retain
input bytes which was used to produce the frame.
After successful call of nghttp2_session_continue, the application
can continue to call nghttp2_session_mem_recv() to process
additional data.