The mentioned method is also changed to only retrieve http header
informations instead of whole URI content.

Caching fixes

This PR fixes some caching bugs:

* Cached methods have been called locally inside of `CouchDBDao`. The caching proxy was bypassed in these cases. All public methods (even those not involved in caching) are now called on the proxy instance. The here provided implementation is not optimal since it goes against SoC but can serve as a temporary fix. A better solution might involve AspectJ's weaving.

* A wrong key for cache eviction has been used for `NewAnswerEvent` in `CacheBuster`.

See merge request !2

CacheEvict optimization

Currently, in a lot of places the whole cache is cleared when only a single entry changes. Even completely unrelated caches are sometimes unnecessarily cleared.

This PR adds `key` and `condition` elements to the annotations and replaces the `allEntries` element where possible.

There is still room for additional optimizations but these require some refactoring. For now, the related code locations have been commented on to keep track of them.

The last commit of this PR might be obsoleted later by [GH16](https://github.com/thm-projects/arsnova-backend/pull/16). But for now it should massively increase the effectiveness of answer statistics caching.

See merge request !1

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.

Test was broken by
7d530139.

Removed unused schemas and updated the remaining for Spring 4.1.

At least caching of learning progress responses
did not work since commit a59eb75f on March, 6th.

While the exact origin of this problem is unclear,
it seemed like Spring was creating more than one
databaseDao bean. One of these beans had no
working caching annotations.

These events add more detail about what specific
action just happened.

Unlock is a better representation because this event only
occurs when multiple questions are published. 'New' should
only be used when questions are created.

Check user object against null before accepting feedback via Websocket.