[SPARK-3114] [PySpark] Fix Python UDFs in Spark SQL.

This fixes SPARK-3114, an issue where we inadvertently broke Python UDFs in Spark SQL.

This PR modifiers the test runner script to always run the PySpark SQL tests, irrespective of whether SparkSQL itself has been modified.  It also includes Davies' fix for the bug.

Closes #2026.

Author: Josh Rosen <joshrosen@apache.org>
Author: Davies Liu <davies.liu@gmail.com>

Closes #2027 from JoshRosen/pyspark-sql-fix and squashes the following commits:

9af2708 [Davies Liu] bugfix: disable compression of command
0d8d3a4 [Josh Rosen] Always run Python Spark SQL tests.
[SPARK-3116] Remove the excessive lockings in TorrentBroadcast

Author: Reynold Xin <rxin@apache.org>

Closes #2028 from rxin/torrentBroadcast and squashes the following commits:

92c62a5 [Reynold Xin] Revert the MEMORY_AND_DISK_SER changes.
03a5221 [Reynold Xin] [SPARK-3116] Remove the excessive lockings in TorrentBroadcast