nghttp2_submit_shutdown_notice() is used to notify the client that
graceful shutdown is started.  We expect that after this call, the
server application should send another GOAWAY using
nghttp2_submit_goaway() with appropriate last_stream_id.  In this
commit, we also added nghttp2_session_get_last_proc_stream_id(), which
can be used as last_stream_id parameter.

This commit implements graceful shutdown in nghttpx.  The integration
test for graceful shutdown is also added.