Moving nghttp's numeric_name function to util.cc and using this to generate the address name in HttpServer.cc

Adding an address parameter that allows nghttpd to bind to a non-default address.  Both IPv4 and IPv6 addresses are supported.  In addition with verbose mode enable the address that the webserver binds to is now printed in addition to the port.

Same reported at #87 but at src/shrpx_config.cc src/instead of util.cc

We may later add append(const T &), where we require T for .size() and
.data().  We can coexist former append_cstr and new one.  If we pass
string literal, append(const (&)[N]) is selected.

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.

SSL_write may return error indicating blocking until all given data
are written.  Because of this, it is preferable to update
last_write_time_ after SSL_write regardless of its return value.

There is no application level failure in h1 upstream.  For h2, SPDY
upstreams, don't call DIE(), instead return -1 to delete handler.

.. so that we can avoid the problem that request_start_time_ is 0