Added the field localCorrelationData to MqttPublish for better flow-handling in reactive-APIs

[codepitbull]

Motivation Using the reactive integrations (rxjava2/reactor) results in a situation where after an MqttPublish has been processed a Mqtt5PublishResult is forward in the stream. This object doesn't contain any processing context which makes a few usecases rather unintuitive to implement. In my case I am receiving from Kafka and want to commit AFTER the message has been sent to HiveMQ. For that I need to get the commit-offset to the end of the stream.

Changes To support the above mentioned usecase I added a localCorrelationData (similar to what Kafka is doing in their client-lib). This field is never propagated to HiveMQ but can be used down the streams. In my case I store the Kafka-Commit-Offset in there and use it to trigger the actual commit.

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[cla-bot[bot]]

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[cla-bot[bot]]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @codepitbull on file. In order for us to review and merge your code, please sign our Contributor License Agreement to get yourself added. You'll find the CLA and more information here: https://github.com/hivemq/hivemq-community/blob/master/CONTRIBUTING.adoc#contributor-license-agreement

[pglombardo]

@cla-bot u so lazy

[pglombardo]

@cla-bot check

[cla-bot[bot]]

The cla-bot has been summoned, and re-checked this pull request!

[pglombardo]

Hi all - let's wrap up this PR this week if possible so it doesn't go stale.