Waitress, unlike unicorn, is multi-threaded. As it does not do access logs
by default, the app needs to be wrapped in TransLogger before being passed to
Waitress.
To make the switch, a custom registry is now also used instead of the global REGISTRY
as the default registry for the app. As part of this change, the default
prometheus metrics are then also registered with this new registry.
Closes: #72
Each worker now uses its own registry instead of the global REGISTRY
to avoid duplicate metric registration, since metrics are served
from the database.
