[SPARK-15606][CORE] Use non-blocking removeExecutor call to avoid deadlocks

## What changes were proposed in this pull request?
Set minimum number of dispatcher threads to 3 to avoid deadlocks on machines with only 2 cores

## How was this patch tested?

Spark test builds

Author: Pete Robbins <robbinspg@gmail.com>

Closes #13355 from robbinspg/SPARK-13906.