[SPARK-18631][SQL] Changed ExchangeCoordinator re-partitioning to avoid more data skew

## What changes were proposed in this pull request?

Re-partitioning logic in ExchangeCoordinator changed so that adding another pre-shuffle partition to the post-shuffle partition will not be done if doing so would cause the size of the post-shuffle partition to exceed the target partition size.

## How was this patch tested?

Existing tests updated to reflect new expectations.

Author: Mark Hamstra <markhamstra@gmail.com>

Closes #16065 from markhamstra/SPARK-17064.