dhcpv6: Do not let rebind and renew go crazy due too low T1 and T2

Force minimum T1 and T2 to prevent renew and rebind go grazy if
the lease is not getting renew or rebound.
dhcpv6: Reset last_request to current time with renew and rebind

last_request needs to be set current time also with renew and
rebind or otherwise the values set by
g_dhcpv6_client_get_timeouts will not be changed.

This fixes the issues with __connman_dhcpv6_start_renew and
check_restart comparing incorrect started,expired-values to current
time after successful renew/rebind.