Rework shutdown

This set of patches is intended to make sure that the agent will reliably stop after a reasonable time if it has been requested to, even in some corner cases.

I removed the counting of shutdown requests: One request needs to be enough in case of for example a shutdown of the host or an automated restart.
Arguably not that important, since its just used internally inside this one module, but for consistency, it probably is better to go with UTC everywhere.