Add PersonRepository#findByTeammatesName and use it

[talkingscott]

The guide already refers to findByTeammatesName and its use in main(), but the code does not match the text. This PR adds that method to the repository class and uses it in the CommandLineRunner. I additionally call System.exit(0) from main() as otherwise active threads leave the application running after the return.

[Buzzardo]

I asked Greg Turnquist to review this one, too. We don't usually add an exit call.

[talkingscott]

Messaging with Redis calls exit, which is where I got the idea. I see that Messaging with RabbitMQ calls close: SpringApplication.run(MessagingRabbitmqApplication.class, args).close();. That looks more idiomatic.