[SPARK-26555][SQL][BRANCH-2.4] make ScalaReflection subtype checking …

…thread safe

This is a Spark 2.4.x backport of apache#24085. Original description follows below:

## What changes were proposed in this pull request?

Make ScalaReflection subtype checking thread safe by adding a lock. There is a thread safety bug in the <:< operator in all versions of scala (scala/bug#10766).

## How was this patch tested?

Existing tests and a new one for the new subtype checking function.

Closes apache#24913 from JoshRosen/joshrosen/SPARK-26555-branch-2.4-backport.

Authored-by: mwlon <mloncaric@hmc.edu>
Signed-off-by: Josh Rosen <rosenville@gmail.com>