* Have all the required dependencies in class path which give us a `MessageGroupStore` implementation
* Disable all their auto-configurations in the `ExcludeStoresAutoConfigurationEnvironmentPostProcessor`
* Enable only those which are configured by the `aggregator.messageStoreType` property
* Implement a fully reactive function around an aggregator making an interaction via a pair of `FluxMessageChannel`
* Fix checkstyle issues
* Change the app generator maven plugin to new coordinates
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