Exit cleanly after flushing stdout

Summary:
Using `process.exit()` stops the node process without waiting for the event loop to finish, so when using async i/o, which is what happens when piped, if the output is buffered, the process can terminate before it finishes flushing the buffer.

This means you only get some of the output and the JSON is malformed.

This fixes it by calling `process.exit()` inside the flushed callback.

Reviewed By: passy

Differential Revision: D15624806

fbshipit-source-id: ea540ed5a40fb1811e5b705b190da96c8e54730d

src/dispatcher/server.js

@@ -90,4 +90,5 @@ export default (store: Store, logger: Logger) => {
       server.close();
     });
   }
+  return server.close;
 };