@Override
  AccountMetadataStore.ZKUpdater createNewZKUpdater(Collection<Account> accounts) {
    Objects.requireNonNull(router.get(), "Router is null");
    return new ZKUpdater(accounts);
  }
Return a map from account id to {@link Account} metadata at given version. It returns null when there is any error.
@param version The version of {@link Account} metadata to return.
@return A map from account id to {@link Account} metadata in json format.
@throws IllegalArgumentException When the version is not valid.
ambry-account/src/main/java/com/github/ambry/account/RouterStore.java