remove all pre-existing upgrade listeners

[mcollina]

Fixes #249

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

This is a complete rewrite of the shutdown logic to avoid monkey-patching. It's a semver-major change.
This also includes a behavior change now, as websocket requests received while the server is closing will now receive a 503 like all other requests.