Make ST_distance core kernels (point-point, point-linestring, linestring-linestring) perfectly load balanced.

[isVoid]

Currently, distance computation kernels are not perfectly load balanced. Most kernels follow this pattern: launch the number of threads on one operand and for each thread loop over points/segments of the correponding pair in the other. This can slow down the operation if the data are unbalanced/skewed.

Instead, the kernels should launch the number of thread that's makes one thread computes one pair of point-point/point-segment/segment-segment, then uses atomic operation to aggregate the result. This avoids slow down if the data is skewed.

[isVoid]

This work require further investigation on the performance for load-balanced kernel. According to a recent benchmark , the "loop-segment" kernels performs better than load-balanced kernel if the dataset is not skewed. However, there exists some ground for optimization for load balanced kernel.

Benchmark:
https://gist.github.com/isVoid/99ce3f6425528217da78118ab2653959