These are promoted to public API from src/http2.h

Only stream ID which larger than currently used stream ID is
detected as idle.

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

This makes the callback semantics consistent with the other
frames like HEADERS/PUSH_PROMISE.

Previously, there is inconsistency when on_frame_recv_callback
is called between HEADERS/PUSH_PROMISE and the other frames.
For former case, it is called before header block, in latter
case, it is called after whole frame is received. To make it
consistent, we call on_frame_recv_callback for HEADERS/PUSH_PROMISE
after its frame is fully received. Since on_frame_recv_callback
can signal the end of header block, we replaced on_end_headers_callback
with on_begin_headers_callback, which is called when the reception
of the header block is started.

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.

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.

This is a preparation to add public nghttp2_data struct to
nghttp2_frame union.

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