Revert "group_by() and count_by() no longer return defaultdicts"

This reverts commits 7630ea08a2b1ddeb6eca4a4ca5c310b249193878 and
890e4bc530245533f435f0a1ecf1f1ef1db72365
Test count_by() returns defaultdicts
Remove obsolete NOTEs and TODO for group_by()
Return defaultdict(list) from group_by_keys()
walk_values() now updates defaultdict item factory

New item factory is composition of mapper and old item factory.