Holds diagnostic tables while their owner is down.

Every Mob.Diag.Store table names this process as its ETS heir, so a store owner that dies hands its tables — rows intact and still writable, since they are public — here instead of deleting them. The next owner takes them back with give_back/2. It does nothing else, so there is nothing in it to crash. Started on first use and unlinked, like the owners.