splitter: 1.0.0-vanilla-kafka - Fixed the test "Context Loads"

* Added `spring-kafka-test` as dependency.
* Added `@EmbeddedKafka` to the integration test.
* Configured the test to support transactions (replication factor == 1).
* Fixed the `SplitterStreamProcessor`: the application logic is executed
  in a background thread, because it blocks the startup of Spring Boot
  otherwise.