Backport KAFKA-8819 and KAFKA-8340 to 1.1 and 1.0 branches

Both KAFKA-8819 and KAFKA-8340 identified bugs within the Kafka Connect plugin classloading mechanism, but the one PR for those could only be backported as far back as the `2.0` branch. I tried backporting to `1.1`, but there were quite a few compilation errors, and I think it's better to have gharris1727 create a new PR targeted to the `1.1` branch.

And since we're close to the 2.4 release, I created this separate issue to track on the older versions.

When this is resolved, we should probably update the fix versions for KAFKA-8819 and KAFKA-8340.