dhcpv6: add a minimum valid lifetime for IA_PD updates

Prefix delegation is removed when valid lifetime is set to 0,
which will result in current prefix assignments to be preserved
in odhcpd as long as their previously configured valid lifetime
allows it. When IA_PD valid lifetime is set to a small value iso 0,
netifd will update old IA_PD assignments, thus allowing removal
of the old prefixes from downstream interfaces.

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>