Clean up a moved channel's frozen copy at its hosting service #528

Found while building #499 and #500.

What happens

A move out of a hosting service (toLocal, or toCompute from a hosted channel since #522) freezes the channel at its service and keeps the frozen object there. Deleting the channel later doesn't remove that copy, and the service refuses a plain delete of a frozen channel. While testing, three such copies had to be unfrozen and deleted by hand.

Expected

• Deleting a channel that once lived at a hosting service also removes its frozen copy there, or
• the move removes the frozen copy once the destination has confirmed activation.

Either way, a delete that leaves a frozen copy behind should not fail silently.

Dogfooding: #5