The `aggregatorFunction` has a flaw subscribing and emitting
data from a source `Flux` too early: from a `doOnSubscribe()`
callback which is really called *before* the real subscription is
registered in the target `Publisher`
Change the logic to the `doOnRequest()` which happens already
after subscription is registered in the `Publisher`
**Cherry-pick to `2020.0.x`**
* 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
