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.