Named queues

Introduced named queues as discussed in #8.

### Highlights

* Instead of having a queue per `vnode`, there is now a queue per named queue on each `vnode`.
* The `claim` and `count-queue` calls now take a `queue` argument.

Right now there is no garbage collection of named queues that are no longer used, so the overhead of a `ConcurrentSkipListSet` will be around until the process is restarted.

Fixes #8.