PREF64 Default lifetime should be 3 * MaxRtrAdvInterval, not just MaxRtrAdvInterval,

[furry13]

See https://www.rfc-editor.org/rfc/rfc8781.html#name-scaled-lifetime-processing

Setting it to MaxRtrAdvInterval means that if the host loses a single RA, the PREF64 lifetime might get to zero before the next RA comes. Mobile phones actually tend to drop some RAs to save the battery. It leads to scenarios when a phone loses PREF64 (and therefore CLAT stops) from time to time (this is what we observed in a real deployment).

A PR is on its way.

[furry13]

#46