Agent Safe Shutdown

[fredo]

created out of #1757

When the agent is shut down, it should

• stop filling new requests
• claim all outstanding fills
• only if all fills are claimed shut down

I am not in favor of taking challenges into account. It's more about the most time critical component, which is the claiming part. Anything else will always have 24 hours time, and we require to be up and running at least once in the timeframe of 24 hours