Use HTTP shutdown instead

Summary: Use the newly exposed HTTP shutdown API. It is simpler.

Reviewed By: antonk52

Differential Revision: D49499264

fbshipit-source-id: 2d81db1d1a66c0b7550ee1245e51d8f1a8671aa6

desktop/flipper-server-core/src/server/startServer.tsx

@@ -151,7 +151,7 @@ async function startHTTPServer(config: Config): Promise<{
   const socket = attachWS(server, config);
 
   exitHook(() => {
-    console.log('Shutdown server');
+    console.log('[flipper-server] Shutdown HTTP server');
     server.close();
   });