Fix/invalidation of resource cache

[white-gecko]

Now the cache entries for resource descriptions are tagged with the resource and model IRIs. Further the two methods are moved to the QueryCache class, so they can be properly invalidated if the regarding invalidation methods are call on the QueryCache object.

[pfrischmuth]

Please rework the test to be part of Erfurt_StoreTest, e.g. by creating a markTestNeedsCache method or something.

Also if it needs an external dependency like a cache backend, pleas consider a integration test instead of a unit test.