[Colossus] Graceful shutdown

Colossus operators will often need to temporarily stop their nodes. This can be for a number of reasons including maintenance or software update. Currently they have no other option other than just doing a hard shutdown of the service. That means that any ongoing uploads will be interrupted and will fail. This can be very frustrating to users with slow connections who could have been trying to upload for past 10-20 minutes for example.

We should introduce some graceful shutdown procedure in which Colossus would not accept any new uploads but would let the ongoing ones finish before completing the shutdown.

This procedure would probably need some timeout or an opt-out so operators don't have to wait very long for their node to shutdown.