wait for active downloads during shutdown

currently, during shutdown, all tcp sockets/connections are closed immediately.

consider only closing the listening socket, and let client connections linger for some time, until all active downloads have had a chance to finish.

this would be useful during upgrades; could start shutting down the old instance, then launch the new one, and let the old one expire on its own when ready. But there's a couple things to keep in mind,

* ongoing uploads must be terminated immediately to avoid multiple instances writing into the same target files
* kill background tasks (up2k snapshots/rescans etc.) so they don't trample eachother
* check if there's any interesting opportunities for sqlite deadlocks