remote-subnet4-set inconsistent work when id/subnet is duplicated.

1. Kea is started without any subnet configured.
2. execute command remote-subnet4-set:

    {"command": "remote-subnet4-set",
     "arguments": {"subnets": [{"interface": "enp0s9",
                                "subnet": "192.168.50.0/24",
                                "pools": [{"pool": "192.168.50.1-192.168.50.100"}],
                                "id": 5}],
                   "server-tags": ["abc"],
                   "remote": {"type": "mysql"}
                   }
     }

3. Kea will be configured correctly with subnet 192.168.50.0/24 with id 5.
4. Let's now execute command remote-subnet4-set with different subnet but with the same value of id:

    {"command": "remote-subnet4-set",
     "arguments": {"subnets": [{"interface": "enp0s9",
                                "subnet": "192.168.51.0/24",
                                "pools": [{"pool": "192.168.51.1-192.168.51.100"}],
                                "id": 5}],
                   "server-tags": ["abc"],
                   "remote": {"type": "mysql"}
                   }
     }

  {
    "arguments": {
      "subnets": [
        {
          "id": 5,
          "subnet": "192.168.51.0/24"
        }
      ]
    },
    "result": 0,
    "text": "IPv4 subnet successfully set."
  }

mysql> select * from dhcp4_subnet;
+-----------+-----------------+---------------+------------------+------------+----------------+--------------+-----------+-----------------+---------------------+-------------+--------------+-------+-------------+------------------------+------------------+-----------------+---------------------+--------------+----------------+
| subnet_id | subnet_prefix   | 4o6_interface | 4o6_interface_id | 4o6_subnet | boot_file_name | client_class | interface | match_client_id | modification_ts     | next_server | rebind_timer | relay | renew_timer | require_client_classes | reservation_mode | server_hostname | shared_network_name | user_context | valid_lifetime |
+-----------+-----------------+---------------+------------------+------------+----------------+--------------+-----------+-----------------+---------------------+-------------+--------------+-------+-------------+------------------------+------------------+-----------------+---------------------+--------------+----------------+
|         5 | 192.168.51.0/24 | NULL          | NULL             | NULL       | NULL           | NULL         | enp0s9    |               1 | 2019-02-19 06:05:39 |        NULL |         NULL | NULL  |        NULL | [  ]                   |                3 | NULL            | NULL                | NULL         |           7200 |
+-----------+-----------------+---------------+------------------+------------+----------------+--------------+-----------+-----------------+---------------------+-------------+--------------+-------+-------------+------------------------+------------------+-----------------+---------------------+--------------+----------------+

result: there is now just one subnet configured and it's 192.168.51.0/24 so previous one was overwritten without warning/notification

5. But when second command will duplicated subnet kea will respond with a bit cryptic error:

{
  "result": 1,
  "text": "unable to execute for <INSERT INTO dhcp4_pool(  start_address,  end_address,  subnet_id,  modification_ts) VALUES (?, ?, ?, ?)>, reason: Cannot add or update a child row: a foreign key constraint fails (`keadb`.`dhcp4_pool`, CONSTRAINT `fk_dhcp4_pool_subnet_id` FOREIGN KEY (`subnet_id`) REFERENCES `dhcp4_subnet` (`subnet_id`) ON DELETE CASCADE ON UPDATE CASCADE) (error code 1452)"
}

id and subnet are two ways of exactly identify record in database so I guess it should work in the same manner. Either override, or return error saying that there is such record in the database.