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.
I believe there can be several tasks in a worker's queue. Do we only make one ReservedResource per worker? If so, what is the value of having this model anymore? Wouldn't it be better to just note on the Worker which resource it is reserved for? If not, this will delete more than one object since it won't be a unique query. It seems like we are making one of these per task_id, so that's probably the most unique attribute to use.