[#597,!323] Use round() when calculating T1/T2 to ensure consistent results

[#597,!323] Use round() when calculating T1/T2 to ensure consistent results

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::setTeeTimes() - added use of round()

src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::setTeeTimes() - added use of round()

src/bin/dhcp6/tests/dhcp6_test_utils.cc
    Dhcpv6SrvTest::checkIA_NA() - replaced used of
    EXPECTED_EQ with logic to add a failure so callers
    can ASSERT_NO_FATAL_FAILURE and pinpoint the invocation.

src/bin/dhcp6/tests/hooks_unittest.cc
    Wrapped calls to checkIA_NA() with ASSERT_NO_FATAL_FAILURE
    Updated expected T1 values to adjust for use of round().