queue: simplify the interface of the Q type
queue/redis: adapt code to the new Q interface
queue/beanstalk: simplifying the queue implementation
queue/beanstalk: fix Get behavior

Always delete the message on Get. When it fails, we put it back in the
queue.
testing/queue: fix FakeQ implementation
app: fix queue related tests
provision/juju: fix queue related tests
provision/docker: fix queue related tests