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().

This commit adds test to submit DATA frame twice and fixes the bug
that 2nd DATA is not sent.

Added test for this crash.

This commit moves frame_type parameter of
nghttp2_data_soruce_read_length_callback in front of stream_id
parameter.  The motivation is that other callback is generally put
frame related parameters first.  To make it consistent, we move
frame_type, which is frame ralted parameter, to the left.

Previously we always call on_frame_send_callback before calling
nghttp2_stream_detach_data() after sending DATA frame.  As a result,
even if DATA frame has END_STREAM, application cannot call
nghttp2_submit_data() in on_frame_send_callback because previous data
is still attached.  This commit makes a change so that
nghttp2_stream_detach_data() is called before on_frame_send_callback
so that application can issue nghttp2_submit_data() in the callback.

Now it returns only stream's available remote window size, without
considering connection level window size.  For connection-level window
size, nghttp2_session_get_remote_window_size() is added by this
commit.  To get old behavior of
nghttp2_session_get_stream_remote_window_size() is use
min(nghttp2_session_get_stream_remote_window_size(),
nghttp2_session_get_remote_window_size()).  The reason of this change
is that it is desirable to know just stream level window size without
taking into connection level window size.  This is useful for
debugging purpose.

Also edited its documentation to make hyperlink works.

nghttp2_on_begin_frame_callback will be invoked when a frame header is
received.

It is not used by library for a while.  It could be used to pass
unsupported extension frames to application, but its interface
requires library to buffer entire frame, which we'd like to avoid.
For unsupported extension frames, we will add new callbacks which does
not require buffering if they are required.

h2-14 now allows extensions to define new error codes.  To allow
application callback to access such error codes, we uses uint32_t as
error_code type for structs and function parameters.  Previously we
treated unknown error code as INTERNAL_ERROR, but this change removes
this and unknown error code is passed to application callback as is.