Move async work out of the Server constructor

Summary:
I feel like doing async stuff here isn't a good idea in general.
But more pressingly, it means you can't immediately call server.close() after new Server(), because the things to close haven't been created yet.

Reviewed By: danielbuechele

Differential Revision: D10488301

fbshipit-source-id: 76ebe91e0c09f353e0bdb9f2e4116757e757abb2

src/__tests__/server.electron.js

@@ -24,6 +24,7 @@ beforeAll(() => {
   }
 
   server = new Server(new LogManager(), mockStore);
+  return server.init();
 });
 
 test('servers starting at ports', done => {