auto reload config when config file changes

[frederik-b]

When used in kubernetes with a config file mounted from a config map,
it would be best to have cooper autodetect changes in a/the config file and auto reload it.

Also: Make a boot param to enable/disable this feature.

[filex]

Yes, we should implement a config reloader!

For me, the main use-case is the local development environment. When I am working in my Couper config it is cumbersome to always restart the docker container (or compose service).

I see the kube use-case as more complicated. If you have multiple replicas of your gateway running, you wouldn't want them to restart at the same time. We would have to implement a very graceful reconfiguration process that doesn't loose any requests.

For local dev setup that isn't so important - allowing for a much simpler implementation for now.