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.
The integration tests reside in integration-tests directory. To run
integration tests, cd integration-tests, then run "go test". Tests
depends on https://github.com/bradfitz/http2.
Tatsuhiro Tsujikawa