Make server multi-threading by using a production WSGI server like `usgi`

[ankostis]

This is a continuation of #97 (x2 unchecked ticks) and a proper fix for #119, which was masked by #123(e9291df),
scheduled for post-2019-release.

Some useful links for launching flask in production setups:

• https://medium.com/@smirnov.am/running-flask-in-production-with-docker-1932c88f14d0
• https://flask.palletsprojects.com/en/1.1.x/tutorial/deploy/