@VisibleForTesting
  public Optional<Startpoint> readStartpoint(SystemStreamPartition ssp, TaskName taskName) {
    Map<String, byte[]> startpointBytes = readWriteStore.all();
    return readStartpoint(startpointBytes, ssp, taskName);
  }
Returns the last {@link Startpoint} that defines the start position for a {@link SystemStreamPartition} and {@link TaskName}.
@param ssp The {@link SystemStreamPartition} to fetch the {@link Startpoint} for.
@param taskName the {@link TaskName} to fetch the {@link Startpoint} for.
@return {@link Optional} of {@link Startpoint} for the {@link SystemStreamPartition}.
        It is empty if it does not exist or if it is too stale.
samza-core/src/main/java/org/apache/samza/startpoint/StartpointManager.java