use a lookup-friendly structure when parsing relay addresses for better performance

For storing relay IP addresses that look like this:

{
    "Dhcp4": {
        "shared-networks": [
            {
                // List of IPv4 relay addresses for which this shared
                // network is selected.
                "relay": {
                    "ip-addresses": [
                        "192.168.56.1",
                        "192.168.56.2"
                    ]
                }
            }
        ]
    }
}

, a typedef std::vector<isc::asiolink::IOAddress> IOAddressList; is used.

It could be a std::unordered_set<isc::asiolink::IOAddress> for better performance. We treat it like a set in every regard. We look up addresses in it. If it contains a duplicate, it throws.

Here's a call graph that shows what percentage addAddress and containsAddress take out of parsing some shared networks with 114 subnets and 32 relay addresses.