[#3356] Avoid FQDN update when lifetime is zero

[#3356] Avoid FQDN update when lifetime is zero

/src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::generateFqdn() - modified to return early if
    the iaaddr valid lifetime is zero.

/src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
    TEST_F(Dhcpv6SrvTest, generateFqdnUpdate)
    TEST_F(Dhcpv6SrvTest, generateFqdnNoUpdate) - new tests

/src/bin/dhcp6/tests/dhcp6_test_utils.h
    class BaseServerTest - now derives from LogContentTest

Added ChangeLog entry