1124570 - Adds two-queue implementation for reserved resources

https://bugzilla.redhat.com/show_bug.cgi?id=1124570

This BZ implements this [design and TODO list](https://fedorahosted.org/pulp/wiki/TwoQueueReservedResourceDesign). Refer to that doc for more info on what is contained in this PR.
The query I was referring to is the query performed by _delete_worker(). This test is making sure it only deletes one worker if two are returned by the query, but I was suggesting that it can only match one worker because it is querying on a unique attribute (right?). If that's all correct, this test doesn't test a real scenario.