Retrieved @Cacheable and changed updateSession method in SessionService, now getting the session by keyword and set all fields that may have changed to a new value

Removed @Chacheable from getSession function, because the Update of a session wont work if only the cache is used for loading the content of a session

Requesting partial lists would interfere with Spring caching so
pagination is handled by the newly introduced PaginationListDecorator
in the controllers. This additionally allows to determine the total
count of items (independent from pagination parameters) which is not
possible with CouchDB.

Added TODOs to keep track of methods still evicting the whole answers
cache for future improvements. Additional refactoring is necessary to
implement these.

Direct method calls bypass the caching proxy if AspectJ weaving is not
configured.

If deleted documents remain in the sort order,
this will crash the application. We simply skip
over any documents we could not load.

With this fix, deleted documents may remain
in the sort order without a problem. Once the
order is updated by the teacher, any deleted
documents will be removed.

This may cause 'lagging' statistics, but that
should not be a problem.