This is the IPv6 branch of the code.

Additional assumptions the tree serializer makes wrt the (IPv4-only) trunk:
  - Services on nodes that need to talk to eachother do so over IPv4.
  - Other than services talking to eachother, there is no need to propagate
    routes to interlink addresses through the network.
  - The prefix length of the network is a /48.
  - Nodes have /64's on their user interfaces. This is needed for
    autoconfiguration to work anyway.
  - This leaves 16 bits of addresses to propagate through the network. 
  - the serializer needs to be able to (de)serialize trees with both IPv4 and
    IPv6 addresses, so it needs to be able to distinguish between the two. one
    bit will do.
  - no one node will ever have more than seven IPv6 user ranges (/64's), so
    three bits will do to describe the number of ranges 
  - no more than 4096 /64's will be deployed for some time to come, so
    actually 12 bits of address are enough to propagate
  - 1 + 3 + 12 = 16 bits to propagate IPv6 addresses
  - 1 + 3 + 20 = 24 bits to propagate IPv4 addresses.

if we ever decide not to propagate interlink addresses for IPv4