[#42] kea-dhcp6 can now parse and use queue-control

[#42] kea-dhcp6 can now parse and use queue-control

src/bin/dhcp6/ctrl_dhcp6_srv.cc
    ControlledDhcpv6Srv::processConfig() - adde packet queue config logic

src/bin/dhcp6/dhcp6_lexer.ll
    Added parsing of queue-control

src/bin/dhcp6/dhcp6_messages.mes
    Added DHCP6_CONFIG_PACKET_QUEUE message

src/bin/dhcp6/dhcp6_parser.yy

src/bin/dhcp6/json_config_parser.cc
    configureDhcp6Server() - now parses "queue-control"

src/bin/dhcp6/parser_context.*
    Added QUEUE_CONTROL

src/bin/dhcp6/tests/config_parser_unittest.cc
    TEST_F(Dhcp6ParserTest, queueControl)
    TEST_F(Dhcp6ParserTest, queueControlInvalid) - new tests

src/lib/dhcpsrv/parsers/queue_control_parser.*
    Revamped to construct an ElementPtr rather than QueueControl