Explicitly close ws in cases of a process exit

Summary:
^

There may be cases whereas the process exits but leaks the socket descriptors.

There's already a hook to close flipper-server.

Reviewed By: passy

Differential Revision: D37550940

fbshipit-source-id: 35004969834bb92bb7cf0f56bda8d7b43c11d6bf

desktop/flipper-server/src/index.tsx

@@ -18,6 +18,7 @@ import open from 'open';
 import {initCompanionEnv} from 'flipper-server-companion';
 import {startFlipperServer, startServer} from 'flipper-server-core';
 import {isTest} from 'flipper-common';
+import exitHook from 'exit-hook';
 
 const argv = yargs
   .usage('yarn flipper-server [args]')
@@ -115,6 +116,10 @@ async function start() {
     'external',
   );
 
+  exitHook(async () => {
+    await flipperServer.close();
+  });
+
   enhanceLogger((logEntry) => {
     flipperServer.emit('server-log', logEntry);
   });