refactor: Moved extracted the `subscribe()`-call from `StorageStrategy`

refactor: Moved extracted the `subscribe()`-call from `StorageStrategy`

* The subscription does no more happen inside the implementations of the
  interface `StorageStrategy`.
* Instead, the methods, that are defined in `StorageStrategy` return the
  created `Flux`.
* The call to `subscribe()` happens in the code, that uses this methods.
* This faciliates feature refinements concerning the asynchronous handling
  of success- and error-cases and so forth.