feat: use gevent instead as a production-ready server

The default server with flask is not production ready, especially for those relying on it in a non-tty environment (means threading, sockets, etc.. are not accessible). This slows down a lot of things. The quick (and correct) fix is to switch to a production-ready threading via gevent (where we enable threads by default on flask). This should improve the performance for everyone.

Resolves #205 (closed).