[4259] kea-dhcp6 now supports replace-client-name modes

[4259] kea-dhcp6 now supports replace-client-name modes

    src/bin/dhcp6/dhcp6_messages.mes
        - Added new log message, DHCP6_DDNS_SUPPLY_FQDN

    src/bin/dhcp6/dhcp6_srv.cc
        - Dhcpv6Srv::processClientFqdn() - modified to support the name
        replacement modes

    src/bin/dhcp6/tests/fqdn_unittest.cc
        - FqdnDhcpv6SrvTest::testReplaceClientNameMode() new method which tests
        a server's handling of a single client packet for a given
        replace-client-name mode.

        - TEST_F(FqdnDhcpv6SrvTest, replaceClientNameModeTest) - new test which
        exercises the permutations of client packets and replace-client-name
        modes.