Fix attributetable and vectorlayercache
* In the attributetable there was a mess with references and pointers, originating from 66fadee8ef.
* QgsVectorLayerCache did sometimes cache features which did not contain all information which needs to be cached and therefore corrupting the cache and leading to incomplete cache hits.
* Add a unit test for the cache problem
* Fix QgsCacheIndexFeatureId
* QgsAbstractCacheIndex::getCacheIterator now produces a QgsFeatureIterator (instead of a list of Feature Ids). This allows to combine a mixed response, partly satisfied by the cache and partly by an additional query to the backend.