## What changes were proposed in this pull request?
`UnsafeExternalSorter.recordComparator` can be either `KVComparator` or `RowComparator`, and both of them will keep the reference to the input rows they compared last time.
After sorting, we return the sorted iterator to upstream operators. However, the upstream operators may take a while to consume up the sorted iterator, and `UnsafeExternalSorter` is registered to `TaskContext` at [here](https://github.com/apache/spark/blob/v2.2.0/core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeExternalSorter.java#L159-L161), which means we will keep the `UnsafeExternalSorter` instance and keep the last compared input rows in memory until the sorted iterator is consumed up.
Things get worse if we sort within partitions of a dataset and coalesce all partitions into one, as we will keep a lot of input rows in memory and the time to consume up all the sorted iterators is long.
This PR takes over https://github.com/apache/spark/pull/18543 , the idea is that, we do not keep the record comparator instance in `UnsafeExternalSorter`, but a generator of record comparator.
close#18543
## How was this patch tested?
N/A
Author: Wenchen Fan <wenchen@databricks.com>
Closes#18679 from cloud-fan/memory-leak.
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