Decide the fate of MyFamily
See discussion on #5565 (moved) and #6676 (moved). It may be that the family declaration is making guard selection worse, and if so, maybe we should not do guard selection in the same way. Having some paths with all three nodes in the same family scares me, though.
We could do better than the current family system using certs as described in prop220. First we should decide whether we want to keep families at all, though.