Negative utilization for large IPv6 subnets

The issue was found during sanity checks for the 1.0 release.

Source: #650 (comment 253282)

The IPv6 utilization is displayed wrong. It has a negative value.

ToDo

• Handle a special -1 value from Kea that means too many addresses/prefixes for uint64 on UI
• Cast the Kea values to uint64 before processing
• Restore the total NAS value from the subnet mask
• Use math/big to calculate utilization of subnets and shared networks