Support for hub and spoke topologies (non directly routable networks)

In some situations (e.g. cluster spanning AWS peered VPCs in a hub/spoke topology), members may not have a direct route to some of the peers.
We have implemented a model defining zones within Memberlist, and effectively restricting some operations based on member's zone id. The logic is specific to a hub and spoke model where all the spokes have aroute to the hub, but none of the spoke have a route to each other.
I am no claiming that our implementation is correct, but I would be willing to work with this group to get this merged if there is any interest.

The issue names no files, tests, or entry points. Start by reviewing Memberlist's handling of peer operations and the proposed zone model for hub-and-spoke networks; done would require an agreed design and validation for peers without direct routes to one another.

Written by the indexing model from the issue text.