SPARK-3025 [SQL]: Allow JDBC clients to set a fair scheduler pool

This definitely needs review as I am not familiar with this part of Spark.
I tested this locally and it did seem to work.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #1937 from pwendell/scheduler and squashes the following commits:

b858e33 [Patrick Wendell] SPARK-3025: Allow JDBC clients to set a fair scheduler pool

(cherry picked from commit 6bca8898a1aa4ca7161492229bac1748b3da2ad7)
Signed-off-by: Michael Armbrust <michael@databricks.com>