Event processor reset with admin api

[trimoq]

We added the new admin API as a third method to reset the tokens. Further, we streamlined the Controller of the example to call the EventProcessorService for the selected method. This allows to separate the things we want to show with the samples (using the three methods to reset the tokens) from the spring specific stuff like the rest controllers.

For the new features, the documentation has been updated, see also the ref-guide updates.

ToDo:

• [ ] It remains to decide if we want to keep the spring specific controller stuff or if we remove that all-together and refer the users to a simple test. This would make playing with the example harder but understanding the relevant parts easier.