#1 Support offloading 'stash' queues

Sometimes we have a need to stash away a large number of items, to queue them back in later. Currently there is no nice way of doing this. We could implemented "stash" queues. For this, we could be simply offload (and load back in) all queues under `PROJECT:stash:*` (this would be easiest to use), or queues named `PROJECT:stash:KEY` with the `KEY`s in `PROJECT:stashes` (less easy to use - perhaps easier to implement?) similar to how we have `PROJECT:priorities` and `PROJECT:filters`.