Example of explicit, graceful shutdown

A new example, or the advanced examples, should demonstrate a clean shutdown with a grace period. It could be triggered by the first SIGINT, or perhaps by an HTTP GET request from the loopback address to a special URL (e.g. "GET /cgi-bin/stop HTTP/1.1").

See: https://github.com/ripple/rippled/commit/9ad2b9be45e768dc2bb088cf9fae71eaf11a7349