INTEXT-121 Flush Async Buffer on Context Stop

JIRA: https://jira.spring.io/browse/INTEXT-121

When using `async` producers, the buffered messages need to be flushed.

Previously, when the context was stopped, such messages were lost.

- Implement `SmartLifecyle` in the producer context and propagate the `stop()` to the underlying producer(s).
- Remove unused attributes from the consumer parser.
- Add a test case to show the buffered message is received after closing the producer.
- Add KafkaRunning JUnit `@Rule`