The libnghttp2_asio library is C++ library built on top of libnghttp2.
Currently, it has server API and easily create HTTP/2 server using
node.js like API calls.  See the example server source code in
examples/asio-sv.cc.  The library depends on Boost::ASIO library.

See previous commit message why we need this.

Previously read and write timeouts work independently.  When we are
writing response to the client, read timeout still ticks (e.g., HTTP/2
or tunneled HTTPS connection).  So read timeout may occur during long
download.  This commit fixes this issue.  This commit only fixes the
upstream part.  We need similar fix for the downstream.

Previously we missed the case where stream->data_item is not deleted
and it caused leak.  Now stream->data_item is properly deleted when
session is deleted.  We decided not to delete data_item in
nghttp2_stream_free() since we need nghttp2_session to decide whether
data_item should be deleted or not there.

With the combination of HTTP/1 upstream and HTTP/2 downstream,
downstream tells SHRPX_NO_BUFFER while connecting to the backend
server.  Previously, we did not call upstream resume_read and upload
was blocked.  This commit now calls upstream resume_read to unblock.
This commit also remove pending output buffer size of Http2Session
when calculating downstream connection's buffer is full.  This is
desirable since we only operate resume_read by stream basis.

Android does not have _Exit.  We detect this and use _exit instead.
clang-3.4 has an issue around undefined reference to
__atomic_fetch_add_4, so we stick to gcc-4.8 for now.

By default, nghttp2 library only handles HTTP/2 frames and does not
recognize first 24 bytes of client connection preface. This design
choice is done due to the fact that server may want to detect the
application protocol based on first few bytes on clear text
communication. But for simple servers which only speak HTTP/2, it is
easier for developers if nghttp2 library takes care of client
connection preface.

If this option is used with nonzero val, nghttp2 library checks first
24 bytes client connection preface. If it is not a valid one,
nghttp2_session_recv() and nghttp2_session_mem_recv() will return
error NGHTTP2_ERR_BAD_PREFACE, which is fatal error.

SETTINGS_MAX_FRAME_SIZE and SETTINGS_MAX_HEADER_LIST_SIZE are 2
missing SETTINGS IDs and now they are shown correctly.

Currently reserved bit is always set to 0.  The addition of reserved
bit is for future extension.

Contributed by G. Vanem

Previously we empties request headers after they are sent to
downstream in order to free memory.  But it turns out that we use
request headers when rewriting location header response field.  Also
user reported that request headers are useful to add new features.
This commits defers the deletion of request headers to the point when
response headers are deleted (which is after response headers are sent
to upstream client).

Even after on_stream_close_callback, Http2DownstreamConnection is
still alive and upstream keeps sending response to the client.  The
consumed bytes are processed normally (data_source_read_callback) and
also we have a code to consume all allocated bytes for
Http2DownstreamConnection object when it is deleted.  This means that
we don't need to and should not consume response data in downstream
on_stream_close_callback.  If we do, we may get assertion error in
Http2DownstreamConnection::resume_read().