Add a command to request the shutdown of the service

In order to be able to restart a compute node, SLURM may need to send a `shutdown' message to the norns service. When this happens, norns should:

1. stop accepting new requests
2. let running tasks complete
3. complete or cancel pending tasks (we have two options here)
4. once all tasks have been served, check whether any tracked namespaces are not empty and fail accordingly if this happens.
5. if no "untracked files" remain, return an "estimated time of shutdown" to command issuer (computed using the BW from both running and pending tasks)