[SPARK-16957][MLLIB] Use midpoints for split values.

## What changes were proposed in this pull request?

Use midpoints for split values now, and maybe later to make it weighted.

## How was this patch tested?

+ [x] add unit test.
+ [x] revise Split's unit test.

Author: Yan Facai (颜发才) <facai.yan@gmail.com>
Author: 颜发才（Yan Facai） <facai.yan@gmail.com>

Closes #17556 from facaiy/ENH/decision_tree_overflow_and_precision_in_aggregation.