KIP-345 Consumer group static membership

[dpkp]

Implements static consumer group membership from KIP-345. Pass group_instance_id to KafkaConsumer() to specify static group membership. Raises FencedInstanceIdError on duplicate consumers if id is reused (in practice the older consumer is typically the one that raises the error). Partition assignors have been updated to consider static members, aligning with the java client implementations.

Note that there are some internal / non-public interface changes that may break code if it has tight dependencies.