public void writeStartpoint(SystemStreamPartition ssp, Startpoint startpoint) {
    writeStartpoint(ssp, null, startpoint);
  }
Writes a {@link Startpoint} that defines the start position for a {@link SystemStreamPartition}.
@param ssp The {@link SystemStreamPartition} to map the {@link Startpoint} against.
@param startpoint Reference to a Startpoint object.
samza-core/src/main/java/org/apache/samza/startpoint/StartpointManager.java