Introduce the idea that queues can be absent for a reason. The traditional absent reason is 'nodedown' and we have a new reason, 'crashed', for when crash recovery has failed and the supervisor has given up. An absent crashed queue is nearly the same as an absent nodedown queue, but we allow for deleting it since it can't be recovered by bringing a node back up.

Currently absent crashed queues are not handled properly by mgmt (they appear to still be there); we might defer that to bug 26151.