[#722] kea-dhcp6 sanity checking of inbound DUID options improved

[#722] kea-dhcp6 sanity checking of inbound DUID options improved

src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::sanityCheckDUID() - modified to attempt to construct
    a DUID instance as final sanity check

src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
    TEST_F(Dhcpv6SrvTest, sanityCheckClientId)
    TEST_F(Dhcpv6SrvTest, sanityCheckServerId) - revamped tests
    to check against max and max + 1

src/bin/dhcp6/tests/dhcp6_test_utils.h
    A little refactoring to ease option creation

src/lib/dhcp/duid.cc
    DUID::DUID(const std::vector<uint8_t>& duid)
    DUID::DUID(const uint8_t* data, size_t len) - updated
    error log to show actual sizes