Add multi request to KafkaRequestReply

[Malandril]

Purpose

This PR adds a new request type to the KafkaRequestReply so that we can receive multiple replies to a request.
This can be useful when you want to send a request, for which multiple consumer groups can answer, or a some service needs to answer with multiple messages.
That was not supported with the Uni<Rep> request.

Implementation

This is a first idea to add the feature, in which the KafkaRequestReplyImpl is modified to use a Multi as a base for all operations instead of the Uni.
If you think this should be a separate, class or if you have any other idea, do not hesitate.

This adds two new methods requestMulti that returns a Multi<Rep>.
I had no idea, for the method names, and we can discuss a better one.
The reply.timeout is applied between each reply, and the ReplyFailureHandler is called for each reply, so one failure on a reply will fail the whole operation.
I didn't write the doc yet.

If you have any input feel free.

[ozangunalp]

That's a very interesting idea. I'll look later today!

[ozangunalp]

@Malandril Thanks for the tests as well!
I've pushed small changes, and exposed a way to complete a pending request.

The upstream consumer request handling is slightly different, but I am unsure of the impact. It was designed to be able to pause the consumer when there aren't any pending requests.