Updates being delayed for longer than 5 seqnos may be legitimate.

This avoids repeatedly sending requests to a source that has just rebooted
and lost its sequence number.

This is in preparation for sub-second hello intervals.

v4mapped ipv6 addresses correspond to ::ffff:0:0/96

If our currently selected nexthop sends us a request, we should not
forward it back, but rather try a different route.