Since recently used headers are in dynamic header table, it is
advantageous to search dynamic table first, saving time to search
through static table.

tiny-nghttpd is HTTP/2 server and its purpose is measure the
performance of nghttp2 library code.  Currently it only accepts direct
HTTP/2 connection only.

Previously when nghttp2_stream_resume_deferred_data() is called,
deferred flags in stream->flags are all cleared.  This is not ideal
because if application returned NGHTTP2_ERR_DEFERRED, and also that
stream is deferred by flow control, then all flags are cleared and
read callback will be invoked again.  This commit fixes this issue.
This changes error condition of nghttp2_session_resume_data().
Previously we return error if stream was deferred by flow control.
Now we don't return error in this case.  We just clear
NGHTTP2_FLAG_DEFERRED_USER and if still
NGHTTP2_FLAG_DEFERRED_FLOW_CONTROL is set, just return 0.

Previously when connection level remote flow control window gets 0, we
mark the stream having DATA frame with
NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL.  When connection level
WINDOW_UPDATE is received, we checks all existing streams, including
closed ones, and call nghttp2_stream_resume_deferred_data().  The
profiler shows this is expensive.

Now we prepare dedicated priority queue for DATA frames.  And we don't
mark stream with NGHTTP2_STREAM_FLAG_DEFERRED_FLOW_CONTROL when DATA
cannot be sent solely due to connection level flow control.  Instead,
we just queue DATA item to queue.  We won't pop DATA item from queue
when connection level remote window size is 0.  This way, we avoid the
expensive operation for all streams when WINDOW_UPDATE is arrived.

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.