KIP-70 / KAFKA-4033: Cleanup inconsistencies in partition assignment semantics

[KIP-70](https://cwiki.apache.org/confluence/display/KAFKA/KIP-70%3A+Revise+Partition+Assignment+Semantics+on+New+Consumer%27s+Subscription+Change) / [KAFKA-4033](https://issues.apache.org/jira/browse/KAFKA-4033) / https://github.com/apache/kafka/pull/1726

This builds on work done for KAFKA-3949 (tracked in #1241) so that should probably get addressed either before or as part of fixing this.