Introduced a new tool `passenger-config detach-process` for gracefully detaching an application process from the process pool.

Has a similar effect to killing the application process directly with `kill <PID>`,
but killing directly may cause the HTTP client to see an error, while using this command
guarantees that clients see no errors.
When shutting down Passenger, immediately shutdown Meteor processes.

Because otherwise we end up waiting indefinitely for its WebSocket connections
to be closed.