Improve ST_Union_Aggr performance

The function ST_Union_Aggr is slow. This is because this function is implemented in the most inefficient way: union geometry one after another.

```scala
class ST_Union_Aggr extends Aggregator[Geometry, Geometry, Geometry] with TraitSTAggregateExec {

  def reduce(buffer: Geometry, input: Geometry): Geometry =

{     if (buffer.equalsExact(initialGeometry)) input     else buffer.union(input)   }

  def merge(buffer1: Geometry, buffer2: Geometry): Geometry =

{     if (buffer1.equals(initialGeometry)) buffer2     else if (buffer2.equals(initialGeometry)) buffer1     else buffer1.union(buffer2)   }

}
```

There are multiple improvements in JTS that improved the performance of union multiple geometry objects:

• Cascaded union: https://lin-ear-th-inking.blogspot.com/2007/11/fast-polygon-merging-in-jts-using.html
• Better overlay implementation (OverlayNG): https://lin-ear-th-inking.blogspot.com/2020/05/jts-overlay-next-generation.html

We should switch to the new `OverlayNGRobust.union` interface to get the best thing JTS provides. This also requires us to keep all reduced geometries in memory, we may maintain a buffer with maximum size, and run OverlayNGRobust.union when the maximum size is exceeded. This will help balancing the speed and memory usage.