XD-2475 MessageBus Producer Batching

JIRA: https://jira.spring.io/browse/XD-2475

Rabbit: Use a `BatchingRabbitTemplate` if batching is enabled.
Kafka: Set properties

Tested with:

    xd:>stream create foo --definition "time | log"
    xd:>stream deploy --name foo --properties "module.time.producer.batchingEnabled=true,module.time.producer.batchSize=6"

Observe messages appear in log in batches of 6.

    xd:>stream undeploy foo

Observe partial batch is flushed.

(Also added RabbitMessageBus test).
Well, to be consistent with the RabbitBus, these are considered 'default' values that can be overridden at the module level; see the application.yml.