Using gunicorn for production, some things to fix

First thing first, here’s the correct command line to use gunicorn:

./venv/bin/gunicorn "argos.server.main:get_application()" -w 4 -k uvicorn.workers.UvicornWorker

(See https://github.com/encode/uvicorn/discussions/1810 for the explanation of "argos.server.main:get_application()" instead of argos.server.main:get_application)

The problem of using gunicorn is that it will use the read_config_and_populate_db fonction for each worker, I think it may become a problem one day. We may use a lockfile, don’t you think, @almet?

• fix command line in doc
• avoid using read_config_and_populate_db for each worker