prepareFollowNameChangesIterator = async host => {
  const existingNames = await E(host).list();
  const changesIterator = makeRefIterator(await E(host).followNameChanges());
  await takeCount(changesIterator, existingNames.length);
  return changesIterator;
}
Calls `host.followNameChanges()`, takes all already-existing names from the iterator,
and returns the iterator.

@param {any} host - An endo host.