public static interface GenericUDAFEvaluator.AggregationBuffer
The interface for a class that is used to store the aggregation result
during the process of aggregation.
We split this piece of data out because there can be millions of instances
of this Aggregation in hash-based aggregation process, and it's very
important to conserve memory.
In the future, we may completely hide this class inside the Evaluator and
use integer numbers to identify which aggregation we are looking at.