LMS-1880 IProcessingPluginTask modified, tests for ProcessDatasetsCommand written.

.../openbis/dss/generic/server/DataSetProcessingContext.java

...msx/cisd/openbis/dss/generic/server/DataStoreService.java

...sd/openbis/dss/generic/server/ProcessDatasetsCommand.java

...dss/generic/server/plugins/demo/DemoProcessingPlugin.java

...ver/plugins/standard/AbstractDropboxProcessingPlugin.java

...s/generic/server/plugins/tasks/IProcessingPluginTask.java

...penbis/dss/generic/server/ProcessDatasetsCommandTest.java

...ic/server/plugins/standard/DataSetCopierForUsersTest.java