Send WINDOW_UPDATE for ignored DATA bytes when manual flow control is enabled

Since we do not call on_data_chunk_recv_callback for ignored DATA
chunk, if nghttp2_option_set_no_auto_connection_window_update is used,
application may not have a chance to send connection WINDOW_UPDATE.
To fix this, we accumulate those received bytes, and if it exceeds
certain number, we automatically send connection-level WINDOW_UPDATE.
Put a limit for total contiguous headers length currently receiving
Don't send more than NGHTTP2_MAX_HEADERSLEN bytes header block
nghttpx: Handle connection flow control for DATA not sent to backend
nghttpx: Handle connection flow control for DATA not sent to frontend
nghttpx: Add ability to Http2Upstream to send RST_STREAM after END_STREAM
nghttpx: Use error_reply instead of RST_STREAM for http2 upstream request
nghttpx: Remove requirement of content-length for HTTP2 upstream POST
nghttpx: Disallow 2nd message in http downstream
nghttpx: Call on_response_headers for response header only in http2 downstream
Add API to check half-closed state for both direction of stream
nghttpd: Add --early-response option

This option is testing the client behavior when it gets response
before sending all request.
nghttp: Add --no-content-length option
nghttpx: Use NGHTTP2_NO_ERROR to close upgraded (tunneled) stream
nghttpx: Pass through NGHTTP2_NO_ERROR from downstream to upstream
nghttpx: Don't add transfer-encoding to CONNECT request