chore: no need for masterSocketServer to start for multiple times at sticky mode

[thorseraq]

Checklist

• [x] npm test passes
• [x] commit message follows commit guidelines

Description of change

When there are >1 workers, it seems https://github.com/eggjs/egg-cluster/blob/74cbb35a8890cdf069f2a98aab288d9804419c70/lib/app_worker.js#L135 will trigger onAppStart() for multiple times, causing startMasterSocketServer() duplicated init, how about considering explicitly constraining it?