Rename nghttp2_data as nghttp2_private_data

This is a preparation to add public nghttp2_data struct to
nghttp2_frame union.
Remove nghttp2_on_data_recv_callback and nghttp2_on_data_send_callback

nghttp2_data is added to nghttp2_frame union. When DATA is
received, nghttp2_on_frame_recv_callback is called. When DATA is
sent, nghttp2_on_frame_send_callback is called.
Code cleanup
Refactor a bit
Update doc
src: Print header emission with stream_id

Since all headers are not always longer available on one
nghttp2_session_mem_recv call, received headers may be interleaved
with transmission log of the other frames. To make it clear that
each header belongs to which stream, each header is printed with
stream_id.