Slightly Change Zone Concept

Right now, zones are used to separate replicas from one another by ensuring that no object, container, or account has two replicas within the same zone. This requires a cluster to have at least a number of zones equal to the replica count (usually 3).

We'd like to change this to be "no replica will be in the same zone as another replica unless there is no other choice." So there could be 3 replicas even if there are only 2 zones.