[SPARK-21563][CORE] Fix race condition when serializing TaskDescriptions and adding jars

## What changes were proposed in this pull request?

Fix the race condition when serializing TaskDescriptions and adding jars by keeping the set of jars and files for a TaskSet constant across the lifetime of the TaskSet.  Otherwise TaskDescription serialization can produce an invalid serialization when new file/jars are added concurrently as the TaskDescription is serialized.

## How was this patch tested?

Additional unit test ensures jars/files contained in the TaskDescription remain constant throughout the lifetime of the TaskSet.

Author: Andrew Ash <andrew@andrewash.com>

Closes #18913 from ash211/SPARK-21563.