Fixes https://github.com/spring-projects/spring-integration-extensions/issues/199
* Upgrade to SI 5.1.x ane SD-Cassandra-2.1.x, Gradle 5.1
* Add Checkstyle to the project
* Make `CassandraMessageHandler` based on the `ReactiveCassandraOperations`
and add reactive capabilities to the handler
* Make return type consistent for all the modes
* Populate `writeOptions` consistently for all the modes
* Migrate tests to AssertJ