The default delegated-len should be /64
There's a substantial discussion happening at IETF (6man and v6ops WGs), related to draft-collink-v6ops-ent64pd and draft-collink-6man-pio-pflag-00.
The leading perspective is that the recommended prefix length for delegation should be /64. This is pretty major change of opinion for Google and Android. We should make sure Kea is ready for those new deployments.