Introduces additional parameters to the DistinctCountThetaSketch aggregation function that
give the end-user more control over how sketches are merged.  The defaults are selected
to ensure that the behaviour remains unchanged over the current implementation.

Furthermore, an accumulator custom object is added to ensure that pairwise union
operations are avoided as much as possible.  Instead, sketches can be aggregated
and merged when a threshold is met.

This better aligns to the default nominal values parameter
that is used in the query aggregation function.

Removes intermediate array list to buffer/accumulate sketch
elements.  Instead, inputs are fed directly to the underlying
union.  This ensures that the memory usage of the merge is
kept under control.

…theta-sketch-accumulator

This reverts commit 2ed38f3.

…theta-sketch-accumulator

…theta-sketch-accumulator