Account getAccountById(Short id) {
    return idToAccountMap.get(id);
  }
Gets {@link Account} by its id.
@param id The id to get the {@link Account}.
@return The {@link Account} with the given id, or {@code null} if such an {@link Account} does not exist.
ambry-account/src/main/java/com/github/ambry/account/AccountInfoMap.java