@Override
  protected void handleInit(Context context) {
    this.joinOpSpec.getJoinFn().init(context);
  }
Implementation of a stream-table join operator that first retrieve the value of
the message key from incoming message, and then apply the join function.

@param <K> type of the join key
@param <M> type of input messages
@param <R> type of the table record
@param <JM> type of the join result
samza-core/src/main/java/org/apache/samza/operators/impl/StreamTableJoinOperatorImpl.java