Add app container re-usability

[frikky]

Right now, each app just runs once, before killing the container. The goal of this issue is to make sure a container can be re-used, and a max amount of containers can run.

Missing pieces:

• A "default" state in the SDK, allowing for an app to ask for new jobs when it's available
• The default state should ask the backend / worker for jobs
• A "default" state for workers, being able to reset all variables
• A Worker API to handle new jobs for the container. e.g. by querying like "hey, I'm shuffle tools with authorization X for execution Y, do you have any new jobs for me?"