Added advance override for AvgParentScorer.

For scoring using advance the AvgParentScorer used the advance override for ParentScorer resulting in returning the sum. This fix adds the advancce override to the AvgParentScorer calculating the avg correctly.

This fix mainly fixes issues with the 0.20.x releasse where advance is used when calculating scores for child queries returning the wrong score when using the 'avg' score_type.