[8/8] incubator-quickstep git commit: Initial commit for QUICKSTEP-28 and QUICKSTEP-29.

Initial commit for QUICKSTEP-28 and QUICKSTEP-29. - This PR creates a specialized hash table implementation for aggregation. - A hash table's bucket can store multiple AggregationStates, corresponding to various AggregationHandles in the query. Earlier we created one hash table for each

[8/8] incubator-quickstep git commit: Initial commit for QUICKSTEP-28 and QUICKSTEP-29.

Initial commit for QUICKSTEP-28 and QUICKSTEP-29. - This PR creates a specialized hash table implementation for aggregation. - A hash table's bucket can store multiple AggregationStates, corresponding to various AggregationHandles in the query. Earlier we created one hash table for each