[#2835] Replace preferred-lifetime as needed

[#2835] Replace preferred-lifetime as needed

src/bin/dhcp6/tests/config_parser_unittest.cc
src/bin/dhcp6/tests/get_config_unittest.cc
src/bin/dhcp6/tests/simple_parser6_unittest.cc
    Updated tests

src/lib/dhcpsrv/alloc_engine.cc
    AllocEngine::getLifetimes6() - modified to replace preferred
    lifetime with  valid-lifetime * .625 it's unspecified or
    greater than valid-lifetime

src/lib/dhcpsrv/alloc_engine_messages.mes
    ALLOC_ENGINE_V6_CALCULATED_PREFERRED_LIFETIME - new message

src/lib/dhcpsrv/parsers/simple_parser6.cc
    SimpleParser6::GLOBAL6_DEFAULTS - removed preferred-lifetime

src/lib/dhcpsrv/tests/alloc_engine6_unittest.cc
    Updated tests