Better deserialization of query results through the ExploreClient

Solving https://issues.cask.co/browse/CDAP-384:
Instead of including the type information in every cell of every row of results of a query, the ``ExploreClient`` re uses the information of type contained in the result schema, to better deserialize from Json to ``QueryResult`` objects.

Bamboo build: https://builds.cask.co/browse/CDAP-DUT123-1
No, because in other cases the expected value might really be a ``Double``, so there's just no cast needed.