The number of closed stream to keep is limited by
MAX_CONCURRENT_STREAMS - current active stream.

We need paddings regardless of payload and frame boundary to mitigate
certain attacks.

Since we handles CONTINUATION internally, we don't show FLAG_PAD_HIGH
and PAD_LOW flags of HEADERS in nghttp/nghttpd. We just show the
total paddings in HEADERS + CONTINUATION.

It is easy enough to check the end of incoming data by evaluating
frame->hd.flags & NGHTTP2_FLAG_END_STREAM in on_frame_recv_callback

The maximum frame size including header block is still limited
to NGHTTP2_HD_MAX_BUFFER_LENGTH, which is 32KB.

The nghttp2_submit_{request,response}2 functions are renamed as
nghttp2_submit_{request, response}.

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.

This function is similar to nghttp2_submit_request and the
difference is it takes an array of nghttp2_nv as name/value pairs.
It is useful if name/value pairs is not NULL-terminated in the
application code.

It would be desired to add option to make this limit configurable.
Fix scripted_recv_callback in tests.

Fix the bug that connection-level local window is not updated
for the data is the last part of the stream. For the stream
level window may ignore this, connection-level window must
be updated. Also this change fixes the bug that connection-level
window is not updated for the ignored DATA frames.