Allow queue-abstraction.

We now support Redis/Beanstalkd.
  Bind to loopback adapter only.
  Added abstraction-layer.

  I'd foolishly forgotten to commit this.

  This is just a trivial layer which allows us to swap out the
 use of [redis](http://redis.io/) in the future.  I've added
 beanstalkd support because I know it and trust it, but it's
 not been field-tested yet.