## What changes were proposed in this pull request?
Previously, `RDD.treeAggregate` used `reduceByKey` and `reduce` in its implementation, neither of which technically allows the `seq`/`combOps` to modify and return their first arguments.
This PR uses `foldByKey` and `fold` instead and notes that `aggregate` and `treeAggregate` are semantically identical in the Scala doc.
Note that this had some test failures by unknown reasons. This was actually fixed in e3554605b3.
The root cause was, the `zeroValue` now becomes `AFTAggregator` and it compares `totalCnt` (where the value is actually 0). It starts merging one by one and it keeps returning `this` where `totalCnt` is 0. So, this looks not the bug in the current change.
This is now fixed in the commit. So, this should pass the tests.
## How was this patch tested?
Test case added in `RDDSuite`.
Closes#12217
Author: Joseph K. Bradley <joseph@databricks.com>
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#18198 from HyukjinKwon/SPARK-14408.
5a3371883a