Abstraction for port discovery / server startup

[dsyer]

Currently a WebServer is deemed to have started when it has written a port file. Some apps do not run as a WebServerApplicationContext so they never write the file, but they still have a startup lifecycle, and could probably be persuaded to send a message (e.g. by writing to a file). An abstraction for this might be useful. Cf #44.

[rwinch]

Thanks @dsyer! Can you provide a concrete example of what you are wanting to solve?

[dsyer]

There's a sample in the gRPC project ("grpc-client"). The server is not a web server, so I had to abuse the notation a bit, and write a custom ApplicationListener. Testjars doesn't provide a hook to register it, so it had to be in META-INF/spring.factories.