Implement example test for common rebalance callback scenarios

We need to add an example test to the PlaintextConsumerTest that tests a common ConsumerRebalanceListener use case. For example, create an integration test that invokes the Consumer API to commit offsets in the onPartitionsRevoked callback.

Please develop this test in a reasonably general way with a view to using it as a template from which other tests can be created later. Eventually we will need to have a comprehensive set of tests that cover all the basic use cases.