Validator should decide which layer a node is in

[futurechimp]

Currently, node operators need to manually assign themselves to a mixnet layer using the --layer argument at node startup. This is pretty annoying as it requires human coordination among operators where it's really not necessary.

The validator node should assign nodes to layers automatically and return the proper layered topology.

[futurechimp]

For the moment we've got the client init deciding this randomly, which should be good enough to get a distribution and rough self-balancing of layers. I'll bump this back into the backlog for the moment as we've got more important things to work on for the moment.