With the migration to our CouchDB4J fork, we can
remove all extensions that are no longer needed.

The delayed pi-round change can now be triggered via a simple api call.
After the api call a timed task will be started, which executes the
necessary steps for a round change. 

Previously, a single wrong answer was sufficient for a
question to get a 0% score. With this change, they
are counted as much as a correct answer. The only
difference now is that they reduce the overall
score of a question. See test cases for a
few examples.

The numbers for 'sessions' and 'questions' are now
derived from their underlying elements:
- open/closed for sessions
- lecture/preparation for questions

Removed test cases because they weren't of much use after
the refactoring.

This will result in events being fired at the right time.
Most importantly, this ensures event-specific caching
annotations to work correctly.

This also fixes a test error.

Caching was too aggressive. However, the database
has been asked for session info even if there
were no actual sessions! A simple check now saves
four immediate CouchDB requests.