* The audit worker wants to get items from the queue and process them.
* The audit chore wants to create new queues and swap them in when the
old queue has been processed.
This change adds a "Queues" struct which handles the concurrency
issues around the worker fetching a queue and the chore swapping a new
queue in. It simplifies the logic of the "Queue" struct to its bare
bones, so that it behaves like a normal queue with no need to understand
the details of swapping and worker/chore interactions.
Change-Id: Ic3689ede97a528e7590e98338cedddfa51794e1b
