void update(DrainMessage eos, SystemStreamPartition ssp) {
    DrainState state = drainStates.get(ssp);
    state.update(eos.getTaskName());
  }
Update the state upon receiving a drain message.
@param eos message of {@link DrainMessage}
@param ssp system stream partition
samza-core/src/main/java/org/apache/samza/operators/impl/DrainStates.java