1102393 - Rework how we select the queue for new reservations.

This commit alters the data models for keeping track of our Celery workers. We used to track them with the model AvailableQueue, and this commit
renames that model to Worker. This commit also removes the tracking of the number of reservations by Worker, relying instead on tracking them with
the ReservedResources only. This should resolve two bugs[0][1].

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1102393
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1102390
How about inserting another element to the tuple, in position 1, that is a random int? That way ties will be broken in a random way, instead of being broken based on sorting of worker objects. Otherwise, as a system's work load goes up and down, it will follow a predictable pattern of which workers get assigned first.

Speaking of sorting worker objects, is it safe to do that? How are they sorted?