[SPARK-17396][CORE] Share the task support between UnionRDD instances.

## What changes were proposed in this pull request?

Share the ForkJoinTaskSupport between UnionRDD instances to avoid creating a huge number of threads if lots of RDDs are created at the same time.

## How was this patch tested?

This uses existing UnionRDD tests.

Author: Ryan Blue <blue@apache.org>

Closes #14985 from rdblue/SPARK-17396-use-shared-pool.