kea accept out-of-subnet reservations

This configuration is allowed by Kea:

{
    "id": 2,
    "interface": "enp0s8",
    "option-data": [
        {
            "code": 22,
            "data": "2001:db8::1,2001:db8::2",
            "name": "sip-server-addr",
            "space": "dhcp6"
        }
    ],
    "pools": [
        {
            "option-data": [
                {
                    "code": 40,
                    "data": "3000::66,3000::77",
                    "name": "pana-agent",
                    "space": "dhcp6"
                }
            ],
            "pool": "2001:db8:2::1-2001:db8:2::ffff:ffff:ffff:ffff"
        }
    ],
    "reservations": [
        {
            "hostname": "reserved-hostname",
            "hw-address": "00:0c:01:02:03:0a",
            "ip-addresses": [
                "3000:2::1"
            ]
        },
        {
            "hostname": "reserved-hostname",
            "hw-address": "00:0c:01:02:03:0b",
            "ip-addresses": [
                "3000:2::2"
            ]
        },
        {
            "hostname": "reserved-hostname",
            "hw-address": "00:0c:01:02:03:0c",
            "ip-addresses": [
                "3000:2::3"
            ]
        },
        {
            "hostname": "reserved-hostname",
            "hw-address": "00:0c:01:02:03:0d",
            "ip-addresses": [
                "3000:2::4"
            ]
        },
        {
            "hostname": "reserved-hostname",
            "hw-address": "00:0c:01:02:03:0e",
            "ip-addresses": [
                "3000:2::5"
            ]
        }
    ],
    "subnet": "2001:db8:2::/64"
}

and Kea assign such out-of-subnet reservations. That leads to a problem if it's working in HA mode:

HA_LEASE_UPDATE_FAILED duid=[00:03:00:01:00:0c:01:02:03:08], tid=0xfa05: lease update to server2 (http://192.168.59.3:8000/) failed: The address 3000:1::4 does not b
elong to subnet 2001:db8:1::/64, subnet-id=1, error code 1

Related #158 (closed)