LMS-1544 Improved performace of listImages and listFeatureVectors.

...cisd/openbis/plugin/screening/server/ScreeningServer.java

...in/screening/server/logic/FeatureVectorDatasetLoader.java

...bis/plugin/screening/server/logic/ImageDatasetLoader.java

...bis/plugin/screening/server/logic/PlateDatasetLoader.java

...enbis/plugin/screening/server/logic/ScreeningApiImpl.java

...openbis/plugin/screening/server/logic/ScreeningUtils.java

...s/plugin/screening/server/logic/ScreeningApiImplTest.java