Prohibit RemoveChannel from deleting a channel being listened to

[gavin-norman-sociomantic]

sociomantic-tsunami/dhtproto#10 discusses problems around ending a DHT Mirror request when the channel being mirrored is deleted. The same argument applies to the DMQ Consume request.

Discussion led to the idea that it should actually be illegal to remove a channel that is being listened to. RemoveChannel is used in two cases:

1. By accident.
2. Deliberately, to remove a channel which is no longer in use.

In both cases, the request should definitely not work on a channel that's currently in use.