Add expression key support for GroupReduce operators

The GroupReduceOperator did not work with DataSets, which have been
grouped by expression keys like groupBy("myField").
[FLINK-758] Add count operator to grouped DataSets

Adds support for count() on grouped DataSets, which will be translated
to a non-combinable GroupReduceFunction. If the grouping is done on an
empty DataSet, e.g. after a filter-all, the result of the count will be
empty (since there are no groups to count).