Re-implement circusweb with tornadoweb

A big refactoring of circusweb.
There is still some work we want to do but we can start the review.

 - Drop gevent
 - Make circus ØMQ call asynchronous
 - Prepare implementation for multi-endpoint simultaneous connections (just need to update templates)
 - Implement multiuser for circusweb
 - Global flake8 compliancy
 - Remove legacy code