src/lib/dhcpsrv/network.*
    Network::RelayInfo
    - modified to support a list of IP addresses
    - added methods:
        addAddress(const asiolink::IOAddress& addr)
        bool hasAddresses()
        bool containsAddress(const asiolink::IOAddress& addr)
        +const IOAddressList& getAddresses()

    Network
    - added wrapper methods for convenience:
        addRelayAddress(const asiolink::IOAddress& addr)
        bool hasRelays()
        bool hasRelayAddress(const asiolink::IOAddress& addr)
        const IOAddressList& getRelayAddresses()

    - toElement() - modified to output ip-addresses list

    Updated the following accordingly:
        src/lib/dhcpsrv/cfg_subnets4.cc
        src/lib/dhcpsrv/cfg_subnets6.cc
        src/lib/dhcpsrv/parsers/dhcp_parsers.cc
        src/lib/dhcpsrv/subnet.cc
        src/lib/dhcpsrv/tests/cfg_shared_networks4_unittest.cc
        src/lib/dhcpsrv/tests/cfg_shared_networks6_unittest.cc
        src/lib/dhcpsrv/tests/cfg_subnets4_unittest.cc
        src/lib/dhcpsrv/tests/cfg_subnets6_unittest.cc
        src/lib/dhcpsrv/tests/dhcp_parsers_unittest.cc
        src/lib/dhcpsrv/tests/shared_network_unittest.cc
        src/lib/dhcpsrv/tests/subnet_unittest.cc