Replace Vert.x Kafka client with native Java Kafka client in the sink endpoint

[ppatierno]

This PR is about replacing the usage of the Vert.x Kafka client on the sink endpoint (consumer side of the bridge) with the native Java Kafka client (provided by Apache Kafka upstream project).

It provides the following things:

• using CompletableFuture(s) based pattern to run consumer requests asynchronously (as today it's done by Vert.x)
• removing the Vert.x instance from the sink/consumer side
• replacing Vert.x Kafka related classes, i.e. KafkaConsumer, KafkaConsumerRecord, and more with the corresponding native ones from the Java Kafka client

As additional note, I decided to start the async operations (via CompletableFuture.runAsync and similar) in the higher level class HttpSinkBridgeEndpoint because, as already raised by Tom in the producer side, we could end at some point to remove the SinkBridgeEndpoint class, having just one. This is also the same behaviour as the producer/source endpoint.

Anyway, there are still some Vert.x structures in place like the ones handling JSON, Buffer(s) and Handler(s).

[scholzj]

LGTM. But I only run through it quickly ... should be probably also reviewed by Tom ;-)

[ppatierno]

Thanks @scholzj ... yeah Tom already had a first pass. @tombentley I addressed your comments, any more feedback on this one?

[tombentley]

Left a few nits, but otherwise LGTM.