Apps extending the server app trait get a graceful shutdown. The
Return a server to run
Explicitly request a graceful shutdown of the service.
There is no operational standard for this, but some common
Return a task to shutdown the application.
This task is run as a JVM shutdown hook, or when
org.http4s.server.ServerApp.requestShutdown is explicitly called.
The default implementation shuts down the server, and waits for
it to finish. Other resources may shutdown by flatMapping this