Fixes race condition on yarn install mutex network

[kentaromiura]

Summary

Fixes a race condition we observed in production, to easily reproduce it, I changed the code in onServerEnd to wait any amount of time before resolving:

const onServerEnd = (): Promise<void> => {
  clients.forEach((client) => {
    client.write('closing. kthanx, bye.');
  });
  return new Promise(ok => {
    setTimeout( () => {
      server.close();
      ok();
    }, 20);
  }
};

This change makes it easier to spot the issue
Test plan
run:

./bin/yarn install --mutex network& ./bin/yarn install --mutex network&./bin/yarn install --mutex network&

IIRC The original code was written like that because I observed some issue in relying on the close event on my original implementation that was using a unix socket.
Since then I changed the strategy and we use the network.
I cannot recreate the problem observed with this implementation.
I believe using the correct events is also much cleaner :)

[bestander]

Why do you socket.unref right after connecting?

[bestander]

After a chat: unref just makes the Node program not to hang if socket is the only thing left in the event loop.