[#365,!296] kea-dhcp6 calculates tee times

[#365,!296] kea-dhcp6 calculates tee times

Server calcultates T1 & T2 when enabled.  Does not
include parser support for calculate-tee-times,
t1-percent, or t2-percent

src/bin/dhcp6/dhcp6_srv.*
    Dhcpv6Srv::setTeeTimes() - new method to
    calcualte T1 and T2

    Dhcpv6Srv::assignIA_NA()
    Dhcpv6Srv::extendIA_NA()
    Dhcpv6Srv::assignIA_PD()
    Dhcpv6Srv::extendIA_NA() - modified to call setTeeTimes()

src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
    TEST_F(Dhcpv6SrvTest, calculateTeeTimers)  - new test

src/bin/dhcp6/tests/hooks_unittest.cc
    Removed overriding of Lease::t1_ and t2_ from tests