dhcpv6-ia: add some noise to the T1 and T2 periods

[ldir-EDB0]

Without this, all the clients get the same timeouts and try to renew addresses at the same time. Reduce the T1 & T2 by a pseudo random 0 to 127 seconds, thus the renew attempts are likely to be spread out a bit in time.

What do we think? Sensible? Stupid?

[systemcrash]

Sensible - helps guard against update avalanches.