Long MAC address in reservation crashes KEA when using reservation-get*

When reservation is added by reservation-add with too long MAC address, KEA is not able to retrieve this reservation using reservation-get* commands.

Reservation is added to the database with long address.

To Reproduce

1. Run Kea dhcpv4 with host_cmds hook
2. Make a reservation using reservation-add command with too long MAC address. Example:

"reservation": {
                "hw-address": "11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff",
                "ip-address": "192.168.50.100",
                "subnet-id": 1
            }

3. Server returns:

  {
    "result": 0,
    "text": "Host added."
  }

4. You can check database contents, long MAC is added to the database.
5. Try to get reservation by any of the reservation-get* commands. Example:

        "arguments": {
            "subnet-id": 1
        },
        "command": "reservation-get-all"

6. Kea exits with exception like HOOKS_CALLOUT_EXCEPTION exception thrown by callout on hook $reservation_get_all registered by library with index 1 (callout address 0x7f8675052580): address vector size exceeds MAX_HWADDR_LEN

Expected behavior

Kea should either not accept too long MAC address in the reservation-add command or return the whole long MAC with reservation-get*

Links:
Bug discovered by @tomek here: stork#771 (comment 289578)
Systems test request: isc-private/qa-dhcp#326