[SPARK-3607] ConnectionManager threads.max configs on the thread pools don't work

Hi all - cleaned up the code to get rid of the unused parameter and added some discussion of the ThreadPoolExecutor parameters to explain why we can use a single threadCount instead of providing a min/max.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #3664 from ilganeli/SPARK-3607C and squashes the following commits:

3c05690 [Ilya Ganelin] Updated documentation and refactored code to extract shared variables